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

[Louis Dionne via llvm-commits]

ldionne wrote:

@jdoerfert @AntonRydahl The new CI job I added is failing due to the inability to find `-lomp`: https://github.com/llvm/llvm-project/actions/runs/8457375990/job/23175325016?pr=66968

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