[patch/rfc,rfa:ppc64] Add osabi wildcard support

[Andrew Cagney]

> and the second is to explicitly register both of these architecture variants vis:
> 
>   gdbarch_register_osabi (bfd_arch_powerpc, bfd_mach_ppc, ...
>   gdbarch_register_osabi (bfd_arch_powerpc, bfd_mach_ppc64, ...

plus, I believe, either
+ gdbarch_register_osabi (bfd_arch_rs6000, bfd_mach_rs6k);
or:
+ gdbarch_register_osabi (bfd_arch_powerpc, bfd_mach_rs6k);

Per this post:
http://sources.redhat.com/ml/binutils/2003-10/msg00543.html
rs6000:6000 isn't considered compatible with the generic PPC, and hence 
the OSABI code couldn't find a match.  That lead to the warning.

Andrew