[llvm] [BOLT] Enable cross compilation of runtime libraries (PR #101180)

[Amir Ayupov via llvm-commits]

https://github.com/aaupov requested changes to this pull request.

Sorry, this can't be merged in the current state.

LLVM project does runtime libraries build differently, please check `runtimes` folder for more details. We don't want to introduce dependencies on host cross-compilers all the while Clang can be used for that purpose. We can add the dependency on Clang for cross-target runtimes build if necessary, as we already do for BOLT tests.

CC @petrhosek

https://github.com/llvm/llvm-project/pull/101180