Build libgomp only if we're also building base system GCC

Clang's OpenMP support will emit Intel OpenMP API library calls,
and will therefore require libiomp (or whatever name is settled on).
An up-to-date version of libgomp is included in ports or pkg GCC.
Thus, there is no reason to build base libgomp without base system GCC.

PR: 199979 (exp-run)
Reviewed by: pfg
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2459