Fix preprocessor checks for bn_mul PPC asm

On OS X, neither __powerpc__ nor __ppc__ is defined on PPC64, so the
asm code was only being used on PPC32.
diff --git a/include/polarssl/bn_mul.h b/include/polarssl/bn_mul.h
index 870be37..3b3147f 100644
--- a/include/polarssl/bn_mul.h
+++ b/include/polarssl/bn_mul.h
@@ -285,7 +285,6 @@
 
 #endif /* MC68000 */
 
-#if defined(__powerpc__)   || defined(__ppc__)
 #if defined(__powerpc64__) || defined(__ppc64__)
 
 #if defined(__MACH__) && defined(__APPLE__)
@@ -371,7 +370,7 @@
 
 #endif
 
-#else /* PPC32 */
+#elif defined(__powerpc__) || defined(__ppc__) /* end PPC64/begin PPC32  */
 
 #if defined(__MACH__) && defined(__APPLE__)
 
@@ -456,7 +455,6 @@
 #endif
 
 #endif /* PPC32 */
-#endif /* PPC64 */
 
 #if defined(__sparc__) && defined(__sparc64__)