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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> Should these symbols be publicly visible if linked into a dynamic library? not publicly visible? Or is it underspecified (i.e. do we need to add some extra step to disambiguate).

@lhames, has the clarification regarding the treatment of visibility in XCOFF been sufficient to confirm the proper direction for this patch?

To summarize, it is technically "underspecified", but common (if not generally advisable) practice is to have these symbols be publicly visible if linked into a dynamic library (as opposed to a main executable).

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