[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
> 
> Can you add openmp as a runtime (or project) to the cmake invocation? We want to build our own libomp, not rely on the system.

Surely I need to do more than that? We are not building our own Clang, so I'd have to configure the test suite to use the just-built `omp` library as well, right?

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