MACHINE_CPUARCH isn't defined. I believe that this will cover all
options.
I didn't define it in kern.mk because $M is set to MACHINE_CPUARCH and
then is expanded for the genassym.o rule in kern.post.mk and kern.mk
is included after this, so the expansion isn't quite right. I think
this is a bug in make, but don't have the time to track it to ground
(and even if I did, fixing it would require a MFC of the change to the
very old systems we're targetting with this fix).
f50baba14b