build: Only assign ARCH if not already set
build: Only assign ARCH if not already set

This allows users to specify the build architecture at the command line,
without having to edit the Makefile. For example:

	$ make ARCH=foo all # Builds for 'foo' architecture
	$ make ARCH=bar all # Builds for 'bar' architecture
	$ make all # Builds for default architecture in Makefile

ChrisCummins/euclid,ChrisCummins/euclid,ChrisCummins/euclid