Fix rs6000 compatibility predicate

[Jim Blandy]

Geoff Keating <geoffk@geoffk.org> writes:

> Jim Blandy <jimb@redhat.com> writes:
> 
> > The code as changed by the patch below would make more sense to me:
> > all powerpc machines are supersets of the base rs6000 machine, but
> > there the two lines diverged, so any later model of the rs6000 is
> > incompatible with any later powerpc model.
> 
> No, that's not true.  The actual compatibility graph is much more
> complicated than that.

Well, I'll readily admit I don't know much about PPC family history,
so I'm not the person to (correctly) implement a more detailed graph.
I home someone knowledgeable will do so, but what should I do in the
mean time?  I don't think my patch makes anything worse.