x86: cosmetics: Comment some #endifs for better readability

libavcodec/x86/dsputil_mmx.c

libavcodec/x86/fmtconvert_init.c

libavcodec/x86/rv34dsp_init.c

libavcodec/x86/rv40dsp_init.c

libavcodec/x86/vc1dsp_mmx.c

libavcodec/x86/vp8dsp_init.c

libavresample/x86/audio_mix_init.c

libavutil/x86/cpu.c

libswscale/x86/swscale.c