GCE: Don't .error on unsupported targets

We ingest Makefile.gce even when we're not trying to create GCE images
so we don't want to .error here. Instead, set GCE_ARCH to a dummy
value which should make the problem clear to anyone who attempts to
create GCE images on an unsupported architecture.

Reported by: Jenkins
Fixes: 0a8ecca4e315 ("GCE: Specify the architecture of images")