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

[Johannes Doerfert via llvm-commits]

jdoerfert wrote:

@AntonRydahl does this pass smoke tests in the builder now?

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