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 886fe11..5fe6318 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -46,6 +46,7 @@
    * Restore ability to locally trust a self-signed cert that is not a proper
      CA for use as an end entity certificate. (This had been removed in
      1.3.6.)
+   * Fix preprocessor checks for bn_mul PPC asm (found by Barry K. Nathan).
 
 = PolarSSL 1.3.7 released on 2014-05-02
 Features