support clean/distclean make targets in unconfigured tree

support clean/distclean make targets in unconfigured tree

commit 2f853dd6b9a95d5b13ee8f9df762125e0588df5d moved the error
handling for $(ARCH) not being set such that it applied to all
targets, including clean and distclean. previously these targets
worked even in an unconfigured tree. to restore the old behavior, make
most of the makefile body conditional on $(ARCH) being set/non-empty
and produce the error via a fake "all" target in the conditional
branch for the case where $(ARCH) is empty.