[Flang][OpenMP] Several omp tests failing on macOS

[ceseo]

Several tests are currently failing on macOS due to a hardcoded library extension:

Failed Tests (7):
  Flang :: Examples/omp-atomic.f90
  Flang :: Examples/omp-declarative-directive.f90
  Flang :: Examples/omp-device-constructs.f90
  Flang :: Examples/omp-in-reduction-clause.f90
  Flang :: Examples/omp-nowait.f90
  Flang :: Examples/omp-order-clause.f90
  Flang :: Examples/omp-sections.f90

[ceseo]

I'll submit a fix for this shortly.