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

[Tomasz Kłoczko via llvm-commits]

kloczek wrote:

> @kloczek that is to skip the part that leads to linker error. And do you have the source to the `llvm` directory (https://github.com/llvm/llvm-project/tree/main/llvm)?

As this PR is about building bolt from per subproject dist tar ball of course I haven't.

>  It requires some files there to build the runtime.

I'm not 100% sure but this sounds like not proper separation of the code between llvm and bolt 🤔 

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