[lld] [LLD][COFF] Support marking sections as x86_64 code in ARM64EC object files (PR #135280)

[Jacek Caban via llvm-commits]

cjacek wrote:

I first noticed this feature in .exp files generated by the MSVC linker when building DLLs. It appears to be useful for embedding simple x86_64 assembly code without needing to create standalone x86_64 object files. I'm considering using it for `__guard_dispatch_icall_dummy` in mingw-w64. However, this would also require assembler support to set the appropriate `IMAGE_SCN_GPREL` characteristics flag. An alternative approach would be to extend the mingw-w64 build system to build selected files as x86_64 and move `__guard_dispatch_icall_dummy` into one of those.

In any case, that part seems worth having in LLD for completeness.

https://github.com/llvm/llvm-project/pull/135280