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

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.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9092 cda61777-01e9-0310-a592-d414129be87e