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

[Henry Jiang via llvm-commits]

mustartt wrote:

> Can you give me an answer as to how JITLink fits into BOLT? I am trying to emphasize that the particular usage of JITLink for the purposes of BOLT can have additional complications in the context of AIX (which is not directly tied to XCOFF, but extends to additional linker behaviours).

I can only give an high level overview. @scui-ibm would you mind giving a more in detailed summary?
 
BOLT uses JITLink as an platform independent linker that has control over exactly how the sections are layed out in the final executable in order to minimize icache and dcache misses. 

After parsing the entire call graph and performing all the bolt related optimization, bolt will emit in memory object files for JITLink to resolve the relocations and reordering of the sections. 

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