Fix ABI detection for empty 'os.platform'

[iv-m]

in operator in bjam always returns true if its first argument has no elements[1]. This means that if os.platform is empty (not detected), the construction introduced in commit d039c8e4da79dbc76481236249e22892bfdde047 sets ABI to aapcs on all platforms where os.platform is empty, including, e.g. riscv64, and breaks build there.

This commit refactors the condition to use '=' operator, to make sure that when os.platform is empty we get the default ABI value, and thus fixes build on riscv64.

[1] https://www.boost.org/doc/libs/1_83_0/tools/build/doc/html/index.html#jam.language.flow_of_control

Fixes: d039c8e4da79dbc76481236249e22892bfdde047

[iv-m]

I encountered the problem when I was building boost 1.83.0 for riscv64. Here is the error message from b2:

error: No best alternative for libs/context/build/asm_sources with <abi>aapcs <address-model>64 <architecture>riscv <asynch-exceptions>off <binary-format>elf <context-impl>fcontext <coverage>off <debug-symbols>off <deduced-address-model>64 
<deduced-architecture>riscv <exception-handling>on <extern-c-nothrow>off <inlining>full <link>shared <optimization>off <os>LINUX <pch>off <preserve-test-targets>on <profiling>off <python-debugging>off <python>3.11 <relevant>abi <relevant>ad
dress-model <relevant>architecture <relevant>binary-format <relevant>toolset <rtti>on <runtime-debugging>off <runtime-link>shared <stdlib>native <strip>off <target-os>linux <testing.execute>on <threadapi>pthread <threading>multi <toolset-gc
c:version>13 <toolset>gcc <variant>release <vectorize>off <visibility>hidden <warnings-as-errors>off <warnings>on
    no match: <abi>aapcs <address-model>32 <architecture>arm <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>aapcs <address-model>32 <architecture>arm <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>aapcs <address-model>32 <architecture>arm <binary-format>elf <threading>multi <toolset>qcc
    no match: <abi>aapcs <address-model>32 <architecture>arm <binary-format>mach-o <threading>multi <toolset>clang
    no match: <abi>aapcs <address-model>32 <architecture>arm <binary-format>mach-o <threading>multi <toolset>darwin
    no match: <abi>aapcs <address-model>32 <architecture>arm <binary-format>pe <threading>multi <toolset>msvc
    no match: <abi>aapcs <address-model>64 <architecture>arm <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>aapcs <address-model>64 <architecture>arm <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>aapcs <address-model>64 <architecture>arm <binary-format>mach-o <threading>multi <toolset>clang
    no match: <abi>aapcs <address-model>64 <architecture>arm <binary-format>mach-o <threading>multi <toolset>darwin
    no match: <abi>aapcs <address-model>64 <architecture>arm <binary-format>pe <threading>multi <toolset>msvc
    no match: <abi>sysv <address-model>64 <architecture>loongarch <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>o32 <address-model>32 <architecture>mips1 <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>o32 <address-model>32 <architecture>mips1 <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>n64 <address-model>64 <architecture>mips1 <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>n64 <address-model>64 <architecture>mips1 <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>32 <architecture>power <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>32 <architecture>power <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>32 <architecture>power <binary-format>mach-o <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>32 <architecture>power <binary-format>mach-o <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>32 <architecture>power <binary-format>mach-o <threading>multi <toolset>darwin
    no match: <abi>sysv <address-model>32 <architecture>power <binary-format>xcoff <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>32 <architecture>power <binary-format>xcoff <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>64 <architecture>power <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>64 <architecture>power <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>64 <architecture>power <binary-format>mach-o <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>64 <architecture>power <binary-format>mach-o <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>64 <architecture>power <binary-format>mach-o <threading>multi <toolset>darwin
    no match: <abi>sysv <address-model>64 <architecture>power <binary-format>xcoff <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>64 <architecture>power <binary-format>xcoff <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>32_64 <architecture>power <binary-format>mach-o <threading>multi
    no match: <abi>sysv <address-model>64 <architecture>riscv <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>64 <architecture>riscv <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>64 <architecture>s390x <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>64 <architecture>s390x <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>32 <architecture>x86 <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>32 <architecture>x86 <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>32 <architecture>x86 <binary-format>elf <threading>multi <toolset>intel
    no match: <abi>sysv <address-model>32 <architecture>x86 <binary-format>mach-o <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>32 <architecture>x86 <binary-format>mach-o <threading>multi <toolset>darwin
    no match: <abi>ms <address-model>32 <architecture>x86 <binary-format>pe <threading>multi <toolset>clang
    no match: <abi>ms <address-model>32 <architecture>x86 <binary-format>pe <threading>multi <toolset>clang-win
    no match: <abi>ms <address-model>32 <architecture>x86 <binary-format>pe <threading>multi <toolset>gcc
    no match: <abi>ms <address-model>32 <architecture>x86 <binary-format>pe <threading>multi <toolset>intel
    no match: <abi>ms <address-model>32 <architecture>x86 <binary-format>pe <threading>multi <toolset>msvc
    no match: <abi>ms <address-model>32 <architecture>x86 <binary-format>pe <threading>multi <toolset>embarcadero
    no match: <abi>ms <address-model>32 <architecture>x86 <binary-format>pe <threading>multi <toolset>borland
    no match: <abi>sysv <address-model>64 <architecture>x86 <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>64 <architecture>x86 <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>64 <architecture>x86 <binary-format>elf <threading>multi <toolset>intel
    no match: <abi>sysv <address-model>64 <architecture>x86 <binary-format>mach-o <threading>multi <toolset>clang
    no match: <abi>sysv <address-model>64 <architecture>x86 <binary-format>mach-o <threading>multi <toolset>gcc
    no match: <abi>sysv <address-model>64 <architecture>x86 <binary-format>mach-o <threading>multi <toolset>darwin
    no match: <abi>sysv <address-model>64 <architecture>x86 <binary-format>mach-o <threading>multi <toolset>intel
    no match: <abi>ms <address-model>64 <architecture>x86 <binary-format>pe <threading>multi <toolset>clang
    no match: <abi>ms <address-model>64 <architecture>x86 <binary-format>pe <threading>multi <toolset>clang-win
    no match: <abi>ms <address-model>64 <architecture>x86 <binary-format>pe <threading>multi <toolset>gcc
    no match: <abi>ms <address-model>64 <architecture>x86 <binary-format>pe <threading>multi <toolset>intel
    no match: <abi>ms <address-model>64 <architecture>x86 <binary-format>pe <threading>multi <toolset>msvc
    no match: <abi>ms <address-model>64 <architecture>x86 <binary-format>pe <threading>multi <toolset>embarcadero
    no match: <abi>x32 <address-model>32 <architecture>x86 <binary-format>elf <threading>multi <toolset>clang
    no match: <abi>x32 <address-model>32 <architecture>x86 <binary-format>elf <threading>multi <toolset>gcc
    no match: <abi>x32 <address-model>32 <architecture>x86 <binary-format>elf <threading>multi <toolset>intel
    no match: <abi>sysv <address-model>32_64 <architecture>x86 <binary-format>mach-o <threading>multi
    no match: <abi>sysv <architecture>arm+x86 <binary-format>mach-o <threading>multi

The build continued, but produced binaries, as the binaries lacked ontop_fcontext, jump_fcontext and make_fcontext implementations.

[olk]

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