It would seem that the vast majority of Makefiles should be able
		to share on ${MACHINE_ARCH}. Would it make sense to have the
		default be ${MACHINE_ARCH} and override to ${MACHINE}?

This would break on sparc ports, where MACHINE is either `sparc' or
`sparc64' depending on the CPU and MACHINE_ARCH is either `sparc' or
`sparc64' depending on the ABI in use.

I suppose we could change the 64-bit MACHINE_ARCH to `sparcv9' but
that would cause certain disruption.

Eduardo