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

[Henry Jiang via llvm-commits]

mustartt wrote:

Okay so I had the time to clean up the initial patch to support building simple programs. Here is my plan for the next following few patches:
1. Implement `registerXCOFFGraphInfo` so that actual `-check` can be used and add few more tests to verify the correctness of the block address and final fixups
2. Refactor some of the common PowerPC fixup from the elf portion
3. Implement rest of the work that needs to be done for the TOC
4. Support rest of the commonly used relocation types

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