Add some appropriate bignum typedefs for generic 64-bit systems,

setting BignumInt to 32 bits. gcc defines _LP64 on x86-64 and
presumably on other 64-bit architectures, so I've conditioned my
defines on that in the hope that they won't need redoing for the next
few such architectures.

I've also added a set for _LLP64, but it's untested as yet.

[originally from svn r9092]