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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> Yes, C++ static initializers are on the roadmap. We need special registration for those symbols.

>From our offline discussion, you indicated that your primary interest in JITLink support is to support BOLT.

I don't understand how BOLT uses JITLink, but it seems to me that the requirements associated with C++ static initializer support for JITLink (to collect them and arrange to run them without using `dlopen`/the system loader) is different than the ones needed for BOLT (where the linker already collected them and produced the relevant post-link structures in the module).

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