[llvm] Jump table annotations for Linux (PR #112606)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

+ @maksfb @aaupov IIRC Bolt had to bend over backwards to identify indirect jumps via jump tables.  They probably have a vested interest in this approach of leaving behind relocations that can be consumed.

> Any hints how to do this properly would be much appreciated.

So it's going to be register allocation that decides to spill values to the stack.  Perhaps the register allocators need to look for the existence of this and update the resulting spills.

cc @MaskRay @jyknight @efriedma-quic  

https://github.com/llvm/llvm-project/pull/112606