x86: mmx2 ---> mmxext in comments and messages

doc/optimization.txt

doc/swscale.txt

libavcodec/x86/dsputil_avg_template.c

libavcodec/x86/dsputil_mmx.c

libswscale/rgb2rgb.c

libswscale/rgb2rgb_template.c

libswscale/swscale_internal.h

libswscale/utils.c

libswscale/x86/output.asm

libswscale/x86/rgb2rgb.c

libswscale/x86/rgb2rgb_template.c

libswscale/x86/swscale.c

libswscale/x86/swscale_template.c

libswscale/x86/yuv2rgb.c