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

[John Ericson via llvm-commits]

Ericson2314 wrote:

@pca006132 Yes, one of the gotchas of standalone builds in general is that `<FOO>_LIBRARY_DIR` is a build directory when it is part of the build, but a (previously installed) install directory if it prebuilt / system package.

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@kloczek 

> ```
> [ 52%] Built target LLVMBOLTRuntimeLibs
> ```

Hmm, I wonder if there is an existing bug that `BOLT_ENABLE_RUNTIME=OFF` doesn't work?

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