x86: check for AV_CPU_FLAG_AVXSLOW where useful

libavcodec/x86/dcadsp_init.c

libavcodec/x86/dct_init.c

libavcodec/x86/fft_init.c

libavfilter/x86/af_volume_init.c

libavresample/x86/audio_convert_init.c

libavresample/x86/audio_mix_init.c

libavresample/x86/dither_init.c

libavutil/x86/float_dsp_init.c

libavutil/x86/lls_init.c