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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> 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).

If you mean to consider XCOFF `C_EXT`/`C_WEAKEXT` symbols with no specified visibility as having "Default" Scope (https://llvm.org/docs/JITLink.html#linkgraph) for the purposes of a LinkGraph, then I think that is reasonable.

Aside: The document linked above seems rather silent on the topic of symbol interpositioning.

> 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.

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

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