Power9 CMake fixes

[scothalverson]

Background

• I'm trying to get Draco to build on IBM Power9 systems

Purpose of Pull Request

• This adds two fixes that allow detecting Spectrum MPI and detecting that GFortran is built for PowerPC

Status

• Reference:
  • Pre-Merge Code Review
  • CDash Status
• Checks
  • Double check that this is needed. We are already building/releasing a different branch of Draco on Power9 systems w/o this fix (KT).
  • Travis CI checks pass
  • Code coverage does not decrease
  • Valgrind test passes
  • Toss3 checks pass
  • Trinitite checks pass
  • Code reviewed/approved, sufficient DbC checks, testing, documentation

[KineticTheory]

@scothalverson Thanks for this patch. We are doing a release right now, so I will review these changes as compared to a different set of changes that I have on a branch. Either way, we will fix any build issues wrt rzansel and sierra.

[scothalverson]

Thanks KT. That works for me. Its non-blocking for me, but I figured I wouldn't be the only one working with Power9.

[KineticTheory]

@scothalverson I'm going to close your PR as it is superseded by #637.