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

[Henry Jiang via llvm-commits]

mustartt 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).

It's in the roadmap but it is not the top priority. I'd like to support all the features and relocations implement first, and worry about things like the static initializers and such after.   

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