Revert "Disable MMX for i686 and pentiumpro"

To avoid being burned at the stake by an angry mob, I am forced to
revert this commit.

Originally committed as revision 24949 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2105,14 +2105,8 @@ elif enabled x86; then
             cpuflags="-march=$cpu"
             disable cmov
         ;;
-        i686|pentiumpro)
-            cpuflags="-march=$cpu"
-            enable cmov
-            enable fast_cmov
-            disable mmx
-        ;;
         # targets that do support conditional mov (cmov)
-        pentium[23]|pentium-m|athlon|athlon-tbird|athlon-4|athlon-[mx]p|athlon64|k8|opteron|athlon-fx|core2|amdfam10|barcelona|atom)
+        i686|pentiumpro|pentium[23]|pentium-m|athlon|athlon-tbird|athlon-4|athlon-[mx]p|athlon64|k8|opteron|athlon-fx|core2|amdfam10|barcelona|atom)
             cpuflags="-march=$cpu"
             enable cmov
             enable fast_cmov