[llvm] [JITLink][XCOFF] Setup initial build support for XCOFF (PR #127266)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> Okay so I had the time to clean up the initial patch to support building simple programs. Here is my plan for the next following few patches:
> 
>     1. Implement `registerXCOFFGraphInfo` so that actual `-check` can be used and add few more tests to verify the correctness of the block address and final fixups
> 
>     2. Refactor some of the common PowerPC fixup from the elf portion
> 
>     3. Implement rest of the work that needs to be done for the TOC
> 
>     4. Support rest of the commonly used relocation types

I am not familiar with the capabilities of the JIT linker. After (3), can we expect to be able to call (from a JIT-linked function) a function from the link target process that throws a C++ exception and be able to catch it in the JIT-linked function? I think that It would additionally make sense to observe how unwinding works for a similar scenario with C code.

Also, are C++ static constructors (also applicable to C with the `__constructor__` GNU attribute) on your roadmap?

https://github.com/llvm/llvm-project/pull/127266