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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

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

Not the same as `static` in C. I mean symbols with what C considers external linkage and are (without reference to how the linker is invoked) unknown in terms of whether they are visible in another loadable object or not.

When `clang --shared` is used to link on the platform, these symbols would generally be made visible by default to other loadable objects (that is they would have a `Scope` value of `Default`).

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