[llvm] [CMake] Use shm_open, not clock_gettime, to detect librt (PR #120727)

[Raphael Kubo da Costa via llvm-commits]

rakuco wrote:

@petrhosek thanks for the approval. Can you merge the PR as well? I don't have write access to the repository.

https://github.com/llvm/llvm-project/pull/120727