--std vs __intN

[DJ Delorie]

> The USE macro may not be needed, making them equivalent to 
> defined(MACRO_GIVING_THE_TYPE) seems good enough to me.

I was following the USE_INT128 macro style from before.

The other macros provide type, bitsize, min, max, and umax.

So going from six to five macros really isn't much of a savings in the
code, but it doesn't matter to me which way it is.

> In my opinion, we don't need to preserve that __STRICT_ANSI__ behavior.

That implies testsuite changes though...

> If someone writes __int128 without __extension__, they get a
> pedantic warning. I assume the same will be true with __intN

It should.

> If you have a compiler option so that __int20 is not available (the
> name is not recognized by front-ends),

That's up to the backends to decide, whether they support the
underlying mode or not.  Even __int128 isn't supported if the backend
doesn't support TImode.