Remove unused and unsupported Cyrix's "Extended MMX",

Add SSE3 support.
Patch by Zuxy Meng < zuxy POIS meng AH gmail POIS com >
Original thread:
04/26/06 13:13:
[Ffmpeg-devel] [PATCH] Bug fix,	SSE3 support in i386/cputest.c and dsputil.h

Originally committed as revision 5326 to svn://svn.ffmpeg.org/ffmpeg/trunk