[llvm] [BOLT] Enable standalone build (PR #97130)

[Tristan Ross via llvm-commits]

RossComputerGuy wrote:

Good way to test this is to copy the `cmake`, `bolt`, `third-party`, and `llvm` directories out into their own directory with this PR applied and then build bolt.

https://github.com/llvm/llvm-project/pull/97130