Split OpenMP from Clang in LLVM

As the OpenMP-V implementation requires to be built with nOS-V, we can
split the OpenMP package in a different derivation to prevent rebuilds
of clang. Additionally, as OpenMP-V now can be build alongside the
vanilla OpenMP runtime, we simply build a single openmp derivation with
both runtimes. Only a single build of the clang compiler is now
required.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>
Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>