Adjust build for upstream Mpich and OpenMPI modules on Perlmutter.

[kngott]

Summary

Upstream MPICH and OpenMPI are available now on Perlmutter. To access them:
module load mpich
module load openmpi

With openmpi, srun also requires the flag --mpi=pmix, or equivalent.
(https://docs.nersc.gov/development/programming-models/mpi/openmpi/)

With these adjustments, HeatEquation runs fine, albeit with some errors at start and/or end, which may be srun flag related.

Additional background

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate