1. uses "ARCH" instead of "MACHINE_ARCH"
2. allow "ARCH" to be over-ridable.
3. fix ordering to better follow Mk/bsd.port.mk guidelines
XXX, I don't like setting "--ignorearch", but some ports just would not
install on amd64 w/o it. Even with the 'ARCH' override, installing these
RPM's would complain about the system being the wrong architecture.