Fix top-level targets with read-only OBJDIR.

This also makes it so that top-level build targets do not immediately create
the OBJDIR. Only sub-make targets will do so. This avoids creating object
directories for targets like 'make check-old' or creating unneeded
MACHINE.MACHINE_ARCH directories during 'make tinderbox'.

Reported by: npn, lifanov
Tested by: npn, Mark Millard
Sponsored by: Dell