moving <limits> bits to config/cpu

[Benjamin Kosnik]

Hey y'all, 

I think this approach to <limits> makes the most sense. At the risk of 
opening another can of worms, would it be possible to convert all the 
current limits bits to use this (sane) approach, instead of the current 
hodge podge approach? Or am I missing something?

-benjamin



>         * configure.target: Set LIMITSH for powerpc-*-*.
>         * config/cpu/powerpc/bits/limits.h: New file.