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

[Gergely Bálint via llvm-commits]

bgergely0 wrote:

> Speaking of which, I wonder if `lib/<target>/<lib>` would be sufficient (perhaps having `/Target/` is redundant?
In the original version I just had `lib/<target>` as the new path, but I realized there are already target-specific files under `lib/Target/<target>` so it felt like the right place to store the new files.

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