[OpenMP] Fix 'libc' configuration when building OpenMP

Summary:
Forgot to port this option's old handling from offload. It's not way
easier since they're built in the same CMake project. Also delete the
leftover directory that's not used anymore, don't know how that was
still there.
GitOrigin-RevId: 44f392e999dcf6718d7dceaa7ccb39306b1c1feb
1 file changed