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

[Louis Dionne via llvm-commits]

https://github.com/ldionne requested changes to this pull request.

Thanks a lot for the patch! I think this is really great and I'm glad we're paving the way for new backends being added in the library. Since this is the first such backend, there are some thing to change as we discussed during our live review, but I definitely see a path forward.

Note to self: did not have time to review `omp_offload.h` yet, will do next time.

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