[llvm] [BOLT] Enable standalone build (PR #87196)

[via llvm-commits]

pca006132 wrote:

@kloczek I think the problem is `LLVM_LIBRARY_DIR`. I think you should probably set it to somewhere in the build directory. It should not write to `/usr/lib64` when you are not installing it.

https://github.com/llvm/llvm-project/pull/87196