Auto merge of #129295 - Zalathar:profiler-builtins, r=Kobzol

Build `library/profiler_builtins` from `ci-llvm` if appropriate

Running all of `tests/coverage` requires the LLVM profiler runtime, which requires setting `build.profiler = true`.

Historically, doing that has required checking out the entire `src/llvm-project` submodule. For compiler contributors who otherwise don't need that submodule (thanks to `download-ci-vm`), that's quite inconvenient.

However, thanks to #129116, the downloaded CI LLVM tarball now contains a copy of LLVM's `compiler-rt` directory, which includes all the files needed to build the profiler runtime. So with a little bit of extra logic in bootstrap, we can have `library/profiler_builtins` look for the `compiler-rt` files in `ci-llvm` instead of the `src/llvm-project` submodule.

Author: bors