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/ChangeLog b/ChangeLog
index a174aeb..f51e772 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -63,6 +63,7 @@
    * Fix detection of DragonflyBSD in net.c (found by Markus Pfeiffer)
    * Stricter check on SSL ClientHello internal sizes compared to actual packet
      size (found by TrustInSoft)
+   * Fix preprocessor checks for bn_mul PPC asm (found by Barry K. Nathan).
 
 = Version 1.2.10 released 2013-10-07
 Changes