Don't try to use MIPS32 asm macros on MIPS64

The MIPS32 bn_mul asm code causes segfaults on MIPS64 and failing
tests. Until someone has time to fix this up, MIPS64 platforms should
fall back to the C implementation (which works fine).
diff --git a/ChangeLog b/ChangeLog
index fcdfccc..15ddee0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -28,6 +28,8 @@
    * Fixed testing with out-of-source builds using cmake
    * Fixed version-major intolerance in server
    * Fixed CMake symlinking on out-of-source builds
+   * Bignum's MIPS-32 assembly was used on MIPS-64, causing chaos. (Found by
+     Alex Wilson.)
 
 = Version 1.2.10 released 2013-10-07
 Changes
diff --git a/include/polarssl/bn_mul.h b/include/polarssl/bn_mul.h
index a449d64..870be37 100644
--- a/include/polarssl/bn_mul.h
+++ b/include/polarssl/bn_mul.h
@@ -770,7 +770,7 @@
     );
 #endif /* Alpha */
 
-#if defined(__mips__)
+#if defined(__mips__) && !defined(__mips64__)
 
 #define MULADDC_INIT            \
     asm(                        \