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

[Kristof Beyls via llvm-commits]

kbeyls wrote:

> Thank you for the PR! Added a few polishing comments.

Thank you very much! I followed most of your suggestions and resolved the comments where I don't think there's any further discussion. I left a few comments unresolved where might have anything more to add...

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