[flang][openmp] Fix `not yet implemented intrinsic` message for omp_lib (#71101)

PR#70386 removed hardcoded omp_lib name when checking if it is intrinsic
module procedure reference - but instead relied on bind(c) to avoid
generating error when the implementation is external to module.

However, this change only worked for HLFIR. Make it work for FIR flow
and update the test to exercise that as well.