Toolchain issues with pcrel addressing

[npiggin]

This issue will track some issues with porting the 64-bit kernel to pcrel addressing. Once we have something reasonably working and found no good way to solve these, they should be raised with relevant toolchain projects.

• @notoc annotation required for function call target symbols. Would be nice to have an assembler option that defaults to notoc (and perhaps could have a @toc annotation if necessary).
• Extended inline asm addressing. I haven't found a way to use more complex addressing, let alone pcrel addressing in asm constraints.
• Prefixed instructions require padding to prevent crossing 64 byte boundaries. This causes the assembler to treat delta between symbols containing prefixed instructions as non-constant even if the symbols and instructions are explicitly aligned to avoid padding inserted (e.g., 8 byte alignment for prefix instruction will prevent implicit size change). This prevents them being used in some cases, e.g., in alternate instruction sequences.
• r2 is unused in pcrel code. Could we have an option to permit the compiler to use it?
• mcount branches still generate R_PPC64_REL24 relocations. Possible to get gcc to emit @notoc branches for those?

[npiggin]

• llvm-as does not recognise the paddi extended mnemonic pla (https://github.com/llvm/llvm-project/issues/62373).
• -msoft-float, -mno-altivec, and -mno-vsx all cause clang to emit non-pcrel code (https://github.com/llvm/llvm-project/issues/62372).

[npiggin]

• bfd ld only selects POWER10 stubs for pcrel code if it sees PCREL relocations in the object file (not including NOTOC, arguably it should consider that), otherwise it emits worse compat stubs. Not too likely to bite, but --power10-stubs will force it to use the good stubs so we could consider using that.

[peter-bergner]

r2 is unused in pcrel code. Could we have an option to permit the compiler to use it?

The ELFv2 ABI updated for PCREL defines r2 to be a volatile register when generating PCREL code. I've tasked one of our GCC developers to enable using r2 when we're generating code for pcrel. No additional compiler option will be required, other than -mcpu=power10 (or -mpcrel) to enable pcrel code generation.