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

[Kristof Beyls via llvm-commits]

kbeyls wrote:

> FYI I bisected a test failure to this change:

Thanks for reporting this.
>From the error message, it looks like there is some non-determinism in how BOLT names basic blocks. In all of the test runs I've done on a few systems, the name of the basic block for this test always matched `.LBB{{[0-9]+}}`. In the failure you shared, it seems the basic block was named `.Ltmp0`. I'll be updating the test cases shortly to a regular expression that will match both styles of basic blocks..

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