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

[Jacob Bramley via llvm-commits]

jacobbramley wrote:

LGTM (as a reviewer of the second kind).

> 2. People familiar with the pac-ret hardening scheme and AArch64.

Thanks @kbeyls!

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