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

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

> No, these exist in the build directory as well. 

Build directory layout mirrors source directory layout. But what about the install directory layout? (set `-DCMAKE_INSTALL_PREFIX` and do `ninja install`). I'm talking about installation, and I think Target is probably not wanted there.

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