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

[Tristan Ross via llvm-commits]

RossComputerGuy wrote:

> So what exactly are we getting by having a standalone CMakeFile?

I'm kinda new with maintaining LLVM in Nix but afaik it comes down to code deduplicating and managing install size. I could get someone who's on the Nix LLVM team to give a better explanation as to why we need standalone builds.

> I also tried running cmake directly on llvm-project/bolt with this patch applied, but it fails

Interesting, I don't get that error in my testing nor with Nix.

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