dsputil: Split off IDCT bits into their own context

configure

doc/optimization.txt

libavcodec/Makefile

libavcodec/aic.c

libavcodec/arm/Makefile

libavcodec/arm/dsputil_arm.h

libavcodec/arm/dsputil_armv6.S

libavcodec/arm/dsputil_init_arm.c

libavcodec/arm/dsputil_init_armv6.c

libavcodec/arm/idctdsp_arm.S

libavcodec/arm/idctdsp_arm.h

libavcodec/arm/idctdsp_armv6.S

libavcodec/arm/idctdsp_init_arm.c

libavcodec/arm/idctdsp_init_armv5te.c

libavcodec/arm/idctdsp_init_armv6.c

libavcodec/arm/idctdsp_init_neon.c

libavcodec/arm/idctdsp_neon.S

libavcodec/asv.c

libavcodec/asv.h

libavcodec/asvdec.c