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

[Lang Hames via llvm-commits]

lhames wrote:

Thanks for the extra details @hubert-reinterpretcast.

> @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).

I think so. It looks like we should map `SYM_V_INTERNAL` to `Scope::Local`, `SYM_V_HIDDEN` to `Scope::Hidden`, and anything else to `Scope::Default`.

> Off-topic, but that is likely going to run into trouble due to AIX's generally eager resolution behaviour.

If you have any time to elaborate on this I'd be interested to hear. It sounds like the primary motivation here is Bolt, but it'd be nice to make ORC / JITLink work for AIX JIT use cases too.

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