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

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

> I realized there are already target-specific files under `lib/Target/<target>` so it felt like the right place to store the new files.

My read of this (correct me if I'm wrong) is that `lib/Target/<target>` are directories which exist in the sources. I think traditionally the install directories of library archives like this would live at `lib/<target>` rather than `lib/Target/<target>` -- sources and builds are different.

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