MINGW-packages: gcc-libs in clang envs does not pull in openmp

gcc-libs is provided by libc++ in clang environments, but it does not also pull in openmp (libomp); in GCC toolchain environments OTOH gcc-libs does also contain libgomp…

This is related to https://github.com/msys2/MINGW-packages/pull/9884

The workaround for now is to also explicitly require openmp for those environments, but it would be convenient to have the same behavior for all packages that already require only gcc-libs so the recipes don’t have to be updated? I guess it would be quite a job to figure out which of these dependants actually need the explicit openmp addition for clang.

Alternatively, this can also be handled by some new pseudo-package like it was done w/ cc recently…