Fix rs6000 compatibility predicate

[Geoff Keating]

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.

-- 
- Geoffrey Keating <geoffk@geoffk.org>