[clang] [openmp] [OpenMP] Respect LLVM per-target install directories (PR #83282)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> It seems being installed twice both under `lib` and `lib/x86_64-unknown-linux-gnu`. files are the identical as diff show nothing.

Makes sense, like `add_llvm_library` is implicitly installing it there, then our subsequent `install` call is doing it again. I wonder if there's a way to change that.

https://github.com/llvm/llvm-project/pull/83282