crossgcc: clean up aarch64 target integration

We already have aarch64 targets. Extend the "all" target.

Change-Id: I74d9bf5123695318c15b73c89f170f3ebb20aa80
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7729
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)