Add spack environments for Kokkos and Kokkos-Kernels

[cedricchevalier19]

These environments are used to build an OpenMP enabled stack that can be used to follow some exercises.

It is beneficial for Kokkos-Kernels exercises as Kokkos-Kernels must be built beforehand.

Caveat:

• by default, OpenMP is not available on MacOS, so perhaps providing a threads environment is something to do, but I do not know if we want to advertize this backend.