science/lammps: fix build on GCC architectures

Don't enable OpenMP everywhere, it causes an error with GCC, reported on https://github.com/lammps/lammps/pull/1483

This requires some hackery with ports not supporting options for specific compilers.

OPENMP flag has removed option helpers due to them not being respected otherwise.

PR: 241156
Approved by: yuri (maintainer, earlier versior), tcberner (mentor, earlier version)
Differential Revision: https://reviews.freebsd.org/D21994