This initial commit does not add any specific binary analyses yet, it
merely contains the boilerplate to introduce a new BOLT-based tool.

This basically combines the 4 first patches from the prototype pac-ret
and stack-clash binary analyzer discussed in RFC
https://discourse.llvm.org/t/rfc-bolt-based-binary-analysis-tool-to-verify-correctness-of-security-hardening/78148
and published at
https://github.com/llvm/llvm-project/compare/main...kbeyls:llvm-project:bolt-gadget-scanner-prototype

The introduction of such a BOLT-based binary analysis tool was proposed
and discussed in at least the following places:
- The RFC pointed to above
- EuroLLVM 2024 round table
https://discourse.llvm.org/t/summary-of-bolt-as-a-binary-analysis-tool-round-table-at-eurollvm/78441
The round table showed quite a few people interested in being able to
build a custom binary analysis quickly with a tool like this.
- Also at the US LLVM dev meeting a few weeks ago, I heard interest from
a few people, asking when the tool would be available upstream.
- The presentation "Adding Pointer Authentication ABI support for your
ELF platform"
(https://llvm.swoogo.com/2024devmtg/session/2512720/adding-pointer-authentication-abi-support-for-your-elf-platform)
explicitly mentioned interest to extend the prototype tool to verify
correct implementation of pauthabi.