Begin supporting nvcc+xl on darwin P9.

[KineticTheory]

Background

• While nvcc+xl works fine on ATS-2, we have struggled to get this working on Darwin.
• We were just missing one flag, added here.

Purpose of Pull Request

• Fixes re-git issue #1288

Status

• Reference:
  • Pre-Merge Code Review
  • CDash Status
• Checks
  • Travis/Appveyor 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]

None of our CI will actually test this change.

[alexrlongne]

Great, thanks for finding this.