abuild: fix cross compiler test

Actually abort if a cross compiler is missing, but also handle
subarchitectures (currently: armv4 and armv7 for arm)
properly.

Change-Id: Idf37fb029178df6f2ac029466c66aaa2010bdbd2
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7297
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>