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

[Lang Hames via llvm-commits]

lhames wrote:

> On AIX, symbols may have No Visibility. !Hidden includes symbols with no visibility.

When you say "no" visibility, do you mean local / private scope? Basically the same as `static` in C?

JITLink's relevant concepts are:
1. `Scope`: `Local` = only visible within the current graph; `Hidden` = visible to other symbols in the dylib, but not outside; `Default` = visible outside the dylib.
2. `Linkage`: `Strong` = references to this name should bind this definition (no duplicate strong defs allowed within a dylib), `Weak` = Discarded unless this is the first instance of this symbol.

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

Yep. We need to address that, but just haven't gotten to it yet.

I'll try to catch up on the rest of this discussion later tonight.

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