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

[Henry Jiang via llvm-commits]

mustartt wrote:

> > With AIX ld with the default option `-bautoexp` or an export list that contains the symbol, then it will have visibility `Exported` in the final dynamic library.
> > @hubert-reinterpretcast Does that sound correct?
> 
> As explained in my other response. The property of being exported is separate from the Visibility. Additionally, symbols with "protected" visibility are generally exported.
> 
> Your understanding of `-bautoexp` does not match mine. My understanding of `-bautoexp` is that it is the default only when not linking a shared library (i.e., it is the default when linking a main program). Additionally, it only causes export of symbols with definitions where it is known that said definitions (if exported) will be used by other modules listed as inputs to the link step.

Correction:

For JITLink, I think its reasonable to mimic what the linker would have done for object files linked with ~-bautoexp in the absence of a export list~ using the export list generated by `CreateExportList` or `-bexpall`, which is to mark global symbols with default visibility so it can be resolved from other LinkGraphs.

This is the equivalent of linking with `-bexpall` not `-bautoexp` (my bad).

This is also a problem we are facing with the Rust drivers as well where we don't have access to the export list, and as a result we end up passing `-bexpall` as a temporary fix. 

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