Fix mips64 bignum implementation

This pull request should fix polarssl on mips64 (which currently segfaults).

* Use correct mips64 define (```__mips64___``` has never been defined by GCC, but ```__mips64``` is).
* Added mips64 to the list of arches supporting 64-bit ints.

I was also going to add an assembly implementation of the code in ```bn_mul.h``` as well, but after doing some benchmarks it turns out GCC 4.9 produces faster code with the generic C version than my attempt at an assembly version!