[libcxx] [libcxxabi] [llvm] Adding Separate OpenMP Offloading Backend to `libcxx/include/__algorithm/pstl_backends` (PR #66968)

[Louis Dionne via llvm-commits]

ldionne wrote:

@jdoerfert  Rebase done. There will probably be many small issues because I couldn't build this locally, but I basically resolved the rebasing issues and reformulated the backend on top of the new dispatching mechanism.

https://github.com/llvm/llvm-project/pull/66968