__intN patch 3/5: main __int128 -> __intN conversion.

[DJ Delorie]

> I don't see flag_iso as relevant here (since the macros are in the 
> implementation namespace).  The definitions could reasonably be restricted 
> to c_dialect_cxx (), though, given that they are specifically for use by 
> libstdc++ (and it's easier to add a macro later for C if needed, than to 
> remove one after adding it).

They reflect a previous check in libstdc++ to remove __int128
specializations for non-ISO cases.  Now, though, libstdc++ doesn't
know which types are needed for pointers and which aren't, so the
check was moved to inside gcc, which does know.