Since bsd.prog.mk includes bsd.obj.mk, and thus bsd.subdir.mk, we must
ensure all our bsd.subdir.mk-affecting variables are set before
including bsd.prog.mk. Since sbin's various Makefile.arch files add to
SUBDIR this results in those not taking effect, and presumably we also
end up not having buildworld as parallel as it should be due to the fact
that SUBDIR_PARALLEL was not being set before including bsd.prog.mk.
MFC with: 0a0f7486413c