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

[Kristof Beyls via llvm-commits]

kbeyls wrote:

#128565 (commit [55c76ea](https://github.com/llvm/llvm-project/commit/55c76ea391225bce1e0b1c1eba57bbd846c671a1) ) which just got merged will hopefully fix the bot failures.

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