Fix bsd.subdir.mk-related issues after 0a0f7486413c

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: 0a0f7486413c147d56808b38055c40c64cff61f5
Reviewed by: olivier
Differential Revision: https://reviews.freebsd.org/D31125