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

[Johannes Doerfert via llvm-commits]

jdoerfert 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

Can you add openmp as a runtime (or project) to the cmake invocation? 

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