build: Generalize yasm/nasm-related variable names

Makefile

avbuild/arch.mak

configure

libavcodec/x86/Makefile

libavcodec/x86/dcadsp_init.c

libavcodec/x86/fmtconvert_init.c

libavcodec/x86/h264_qpel.c

libavcodec/x86/hpeldsp_init.c

libavcodec/x86/mpegaudiodsp.c

libavcodec/x86/qpeldsp_init.c

libavcodec/x86/rv40dsp_init.c

libavcodec/x86/vc1dsp_init.c

libavcodec/x86/videodsp_init.c

libavcodec/x86/vp8dsp_init.c

libavcodec/x86/vp9dsp_init.c

libavfilter/x86/Makefile

libavfilter/x86/vf_gradfun_init.c

libavfilter/x86/vf_hqdn3d_init.c

libavresample/x86/Makefile

libavutil/x86/Makefile