Fix mips64 bignum implementation

- Use correct mips64 define (__mips64, not __mips64__).
- Added mips64 to the list of arches supporting 64-bit ints.
diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index 1b40100..ddcc0dd 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -133,7 +133,7 @@
           defined(__ppc64__) || defined(__powerpc64__) || \
           defined(__ia64__)  || defined(__alpha__)     || \
           (defined(__sparc__) && defined(__arch64__))  || \
-          defined(__s390x__) ) )
+          defined(__s390x__) || defined(__mips64) ) )
        typedef  int64_t t_sint;
        typedef uint64_t t_uint;
        typedef unsigned int t_udbl __attribute__((mode(TI)));