Split openmp versions in separate derivations

The openmp derivation provides both libomp and libompv. To avoid
accidentally linking with the wrong library and to avoid the nosv
dependency on libomp, this patch separates each version in a different
derivation.

Also, it adapts the clang wrappers and stdenvs to provide an stdenv per
openmp library where each openmp will be used by default when the
compiler flag "-fopenmp" is used. This eases linking ompv with nixpkgs
libraries, such as blis, that expect openmp to be provided with stdenv.

Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>
Tested-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>