support larger int types

[Ulrich Drepper]

Hey,

I have a question about an issue which might on affect me or it might
not.  In future I'm sure more people will stumble across this.

gcc and other compilers now support for many targets types larger than
long.  This has long been as issue for long long on 32-bit platforms
but now there is __int128.  In arithmetic-heavy code I use it as an
intermediate type before switching to mpz etc and therefore it would
be nice to be able to convert such numbers.  I.e., variants of
mpz_set_si are needed.

I wrote those for my code.  The question is: would these be of
interest for upstream?  I'm perfectly happy to send my code along.

Thanks.