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

[Henry Jiang via llvm-commits]

mustartt wrote:

> 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?

After 4, when we get all the relocation types used in eh_frames, working then we should be able to catch exceptions.

Yes, C++ static initializers are on the roadmap. We need special registration for those symbols. 

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