[patch] change specific int128 -> generic intN

[DJ Delorie]

> Well, it wasn't a hard requirement, it is just that the library has
> to use a more complicated way to get the precision (use (unsigned
> TYPE)(-1) to get the unsigned max and compute the precision from
> that, probably).

We could define macros for the precision too, and we already know max
and min values as macros, it's "just a matter of" exporting that info
to the C++ headers somehow.

> > Would it be acceptable for the compiler to always define a set of
> > macros for each of the intN types?
> 
> What set of macros do you have in mind?

In general, I meant.  They'd be predefined for pretty much every
compile, not just the C++ headers.