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

[Kristof Beyls via llvm-commits]

kbeyls wrote:

@asl FYI: since you mentioned the usefulness of a scanner for checking whether PAuthABI was applied correctly to a binary in your presentation on PAuthABI at the US LLVM dev meeting: The scanner for pac-ret in this PR could be the basis of a scanner that also checks PAuthABI security properties.

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