x86util: Add cpuflags_mmxext alias for cpuflags_mmx2

"mmxext" is a more sensible name and more common in outside projects.

libavutil/x86/x86util.asm

@@ -23,6 +23,8 @@
 ;* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 ;******************************************************************************
 
+%define cpuflags_mmxext cpuflags_mmx2
+
 %include "libavutil/x86/x86inc.asm"
 
 %macro SBUTTERFLY 4