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

[Florian Weimer via llvm-commits]

fweimer-rh wrote:

I think it's reasonable to use `shm_open` as an indicator for a separate `librt` because it was moved to `libc` at the same time as the bulk of the symbols, unlike `clock_gettime`, which was moved much earlier.

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