Really rule chaining is not a particularly expensive task, since
GNU Make caches the directory listing.  However it is easy to
avoid it for most files and for phony targets (one was missing).

After this patch, only "Makefile", "scripts/hxtool" and
"scripts/create_config" attempt to use chained rules.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>