Set MPI environment variables to the compiler wrappers on Cray (#2762)
Set MPI environment variables to the compiler wrappers on Cray (#2762)

* Set MPI[lang] env var to compiler wrappers on cray

Sets MPICC, MPICXX, MPIFC etc to point to the compiler wrappers on Cray systems.
Some packages look for MPIs via environment variables - e.g adios

* Use spec to check for Cray

* os is no longer used.

LLNL/spack,LLNL/spack,TheTimmy/spack,mfherbst/spack,LLNL/spack,iulian787/spack,skosukhin/spack,LLNL/spack,tmerrick1/spack,EmreAtes/spack,mfherbst/spack,krafczyk/spack,lgarren/spack,iulian787/spack,mfherbst/spack,TheTimmy/spack,TheTimmy/spack,krafczyk/spack,skosukhin/spack,iulian787/spack,matthiasdiener/spack,lgarren/spack,skosukhin/spack,LLNL/spack,mfherbst/spack,EmreAtes/spack,lgarren/spack,tmerrick1/spack,tmerrick1/spack,matthiasdiener/spack,EmreAtes/spack,lgarren/spack,tmerrick1/spack,krafczyk/spack,EmreAtes/spack,lgarren/spack,TheTimmy/spack,matthiasdiener/spack,skosukhin/spack,iulian787/spack,skosukhin/spack,EmreAtes/spack,tmerrick1/spack,TheTimmy/spack,krafczyk/spack,mfherbst/spack,matthiasdiener/spack,iulian787/spack,krafczyk/spack,matthiasdiener/spack