[llvm] [BOLT][binary-analysis] Add initial pac-ret gadget scanner (PR #122304)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

I think we're converging on having this PR ready to be merged.
@maksfb : before merging I thought to just check if you see any reason not to merge this PR?

https://github.com/llvm/llvm-project/pull/122304