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

[Shimin Cui via llvm-commits]

scui-ibm wrote:

> I can only give an high level overview. @scui-ibm would you mind giving a more in detailed summary?

I also only have high levels view of the interaction between BOLT and JITLinker. Basically after BOLT transformations, BOLT JITLinkLinker calls jitlink::createLinkGraphFromObject to create a LinkGraph and then calls jitlink::link to link it (section allocation and relocation fixups). There are some details/discussions in this PR (https://reviews.llvm.org/D147544,  [BOLT] Move from RuntimeDyld to JITLink).

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