dsputil: Split off pixel block routines into their own context

configure

libavcodec/Makefile

libavcodec/arm/Makefile

libavcodec/arm/dsputil_armv6.S

libavcodec/arm/dsputil_init_armv6.c

libavcodec/arm/pixblockdsp_armv6.S

libavcodec/arm/pixblockdsp_init_arm.c

libavcodec/asv.h

libavcodec/asvenc.c

libavcodec/dnxhdenc.c

libavcodec/dsputil.c

libavcodec/dsputil.h

libavcodec/dvenc.c

libavcodec/mpegvideo.h

libavcodec/mpegvideo_enc.c

libavcodec/pixblockdsp.c

libavcodec/pixblockdsp.h

libavcodec/pixblockdsp_template.c

libavcodec/ppc/Makefile

libavcodec/ppc/dsputil_altivec.c