Add support for LLD linker and Mac OS platforms

[Scheremo]

Please don't merge before merging changes in GVSoC - my own submodules are linked in .gitmodules and changes are coupled. Also build flow expects a libc implementation in ${PULP_RISCV_LLVM_TOOLCHAIN}/picolibc/riscv. Let's please discuss a canonical libc implementation for SDKs.

This PR supports using the LLD linker with PULP-SDK projects on Linux platforms, and adds support for Mac platforms to run GVSoC and build PULP-SDK with Apple LLVM.

SDK

Changed

• Linker script no longer aliases regions
• Assembly function files correctly annotate section attributes ax and %progbits
• Assembly files are compiled with -fno-inline-functions to ensure correct positioning in binary
• Added -mno-relax flag to LLVM build flow to enable LLD linking
• Added both -z norelro and -fuse-ld=lld flags to linker invocation to force use of lld when LLVM toolchain is set, and avoid linking errors related to unaligned .got and .relro sections -> This should be fixable by either removing .got in the linker script or removing the . ALIGN(4) attribute in the .got section and making it contiguous with the .rodata section.

GVSoC

Changed

• Added conditional compile flags for APPLE platforms which avoid setting RTLD_DEEPBIND for dynamically linked libraries, as this is undefined for Apple Clang and the default behaviour on this platform, anyway.
• Removed all dependencies on Linux-only prctl sys library. This was used to indicate parent exit to child processes to make them suicide. The same functionality is now implemented in POSIX compliant fashion, by spawning a third monitor process, which polls the parent until it closes. Once this happens, the monitor process closes the child process, then itself.
• Removed __STDC_IEC_559__ requirement in flexfloat.h, as it's not really required and trips Apple platforms.
• Fixed broken iterator loop in gvsoc_pulp which lead to undefined behaviour.

Removed

• dpi_chip_wrapper should not be built on Apple platforms - Afaik there is 0 support for EDA tools on Apple platforms, so I just removed it.

Fixed

• Small bug where file ending of dynamically linked libraries was set to ".so". Added conditional compile flag for Apple platforms, which uses ".dylib" instead.
• Set required C++ standard to std=c++17, which is required for LLVM compilation
• Fixed RPATH problems which were worked around by setting a global LD_LIBRARY_PATH export. Dynamic library location is now correctly set via the RPATH variable in dependent binaries, pointing to their final install location.

[Scheremo]

In conjunction with these PRs on GVSoC dependencies: GVSoC PULP GVSoC