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/bn_mul.h b/include/polarssl/bn_mul.h
index e888cc2..ca7d896 100644
--- a/include/polarssl/bn_mul.h
+++ b/include/polarssl/bn_mul.h
@@ -680,7 +680,7 @@
     );
 #endif /* Alpha */
 
-#if defined(__mips__) && !defined(__mips64__)
+#if defined(__mips__) && !defined(__mips64)
 
 #define MULADDC_INIT                    \
     asm(                                \