Make mips64 a separate arch with fast_64bit.

Originally committed as revision 16101 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1322,9 +1322,13 @@ EOF
         enable fast_64bit
         enable fast_unaligned
     ;;
-    mips|mipsel|mips64|IP*)
+    mips|mipsel|IP*)
         arch="mips"
     ;;
+    mips64)
+        arch="mips"
+        enable fast_64bit
+    ;;
     sun4u|sparc64)
         arch="sparc64"
         enable fast_64bit