PowerPC: Define _STRING_ARCH_unaligned

[Roland McGrath]

> I think the common case, in terms of number of architectures, is probably 
> 0.

That is functionally consistent with the current code, where the generic
file omits the macro.  But let us measure rather than guess.  Let someone
(the arch maintainer or whoever gets to it) say for each arch what the
proper value is, and let someone collate all those results.