charm: provide MPI (#8647)
charm: provide MPI (#8647)

* Charm: provide MPI
* add mpicc wrappers
* clarify MPI comment

mfherbst/spack,krafczyk/spack,LLNL/spack,mfherbst/spack,iulian787/spack,LLNL/spack,mfherbst/spack,LLNL/spack,iulian787/spack,LLNL/spack,krafczyk/spack,mfherbst/spack,iulian787/spack,krafczyk/spack,mfherbst/spack,LLNL/spack,krafczyk/spack,iulian787/spack,krafczyk/spack,iulian787/spack