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

[Amir Ayupov via llvm-commits]

aaupov wrote:

> @aaupov what is the requirement for a test plan? Should I give a docker script to run the standalone build or is something like a nix file sufficient?

A reproducer invocation would be enough. Docker script is also welcome, you can use the official Dockerfile (https://github.com/llvm/llvm-project/blob/main/bolt/utils/docker/Dockerfile) as reference.

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