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

[Johannes Doerfert via llvm-commits]

jdoerfert wrote:

@AntonRydahl I tried to rebase this but there are conflicts that I cannot easily resolve.
Can you give it a try please?

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