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/ChangeLog b/ChangeLog
index b13d757..597d144 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -24,6 +24,7 @@
      in 1.2.12).
    * Fix unchecked return code in x509_crt_parse_path() on Windows (found by
      Peter Vaskovic).
+   * Fix assembly selection for MIPS64 (thanks to James Cowgill).
 
 Changes
    * Blind RSA private operations even when POLARSSL_RSA_NO_CRT is defined.