Where are MACHINE_ARCH and LOWER_ARCH defined?

I added an echo to see them in the bsd.prefs.mk and commented out the
section for the OPSYS. They appear to be defined earlier.

Anyways, my problem is my MACHINE_ARCH is i686, but some Makefiles only
check for i386.

I think that it should maybe always be converted, like:

sed -e 's/[456]86/386/'

But do we ever have need to check just for i386 or just i486 (for older
CPUs)?

Or will there ever be a i786?

The mk/bsd.prefs.mk doesn't fix it, because it only changes when
MACHINE_ARCH is "unknown".

My pr is pkg/19789.

   Jeremy C. Reed
   http://bsd.reedmedia.net/