[openmp][NFCI] Move .mod code out of runtimes subdir (#169909)
Extracted out of #169638. The motivation is that we want to build
Fortran module files for device triples (amdgpu-amd-amdhsa and
nvptx64-nvidia-cuda) as well, but the runtimes/ directory is only
included for host devices.
This PR itself should not change anything, including that omp_lib.mod is
only built on host devices triple. Some dependencies used for building
omp_lib.mod are hoisted out of runtimes/ as well. IMHO they all make
sense to hoist, e.g. LIBOMP_VERSION_MAJOR/LIBOMP_VERSION_MINOR should be
usable in the entire OpenMP subproject.