All of the compilers used to build FreeBSD should now default to
-fno-common, so this isn't really necessary anymore -- it was only
added to avoid regressing the -fno-common build before it became the
default everywhere.

For <kmod.mk>, we'll leave that one in place. I didn't actually add
that one, and it's more important that we always have -fno-common in
kmod builds.