Add/improve PPC64 support. On such machines, "uname -m" returns "ppc64,

so recognize them as such.

Originally committed as revision 5728 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -365,7 +365,7 @@ case "$cpu" in
   alpha)
     cpu="alpha"
   ;;
-  "Power Macintosh"|ppc|powerpc)
+  "Power Macintosh"|ppc|ppc64|powerpc)
     cpu="powerpc"
   ;;
   mips|mipsel|IP*)