'0514 libstdc++ fails building powerpc/limitsMEMBERS.cc on rs6000-ibm-aix4.2.0.0

[Benjamin Kosnik]

Gaby, can you look at this please?

thanks,
benjamin

> The use of mknumeric_limits needs  more thought for multilibbed
> environments
> - on AIX. Rainer's recent change makes it clear that the prior build
> problem reported
> was caused by mknumeric_limits failing.
> Maybe if in a multilibed environment, gen-num-limits should fall back to
> using the `native'
> build of mknumeric_limits if the multisubdir version fails.