We don't _always_ need dev-libs/libatomic_ops and it's not accurate to depend on it purely by arch in ebuilds because e.g. some arms will need it and some won't. Perhaps we should just add it to @system for certain profiles?
To be clear, this DOES NOT provide libatomic, but IIRC I've seen it not used on arches anyway, but maybe I'm wrong and I misunderstood or misremembered something at the time? Needs checking..