ringtest: use link-time optimization

By using -flto and -fwhole-program, all functions from the ring implementation
can be treated as static and possibly inlined.  Force this to happen through
the GCC flatten attribute.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>