Move decode_significance_x86() and decode_significance_8x8_x86() to

i386-specific file from cabac.h.
New file is h264-oriented and only included from h264.c
Resolves compilation when configured with --disable-optimizations due to
decode_significance_8x8_x86 using last_coeff_flag_offset_8x8, which is
only defined in h264.c

Originally committed as revision 12838 to svn://svn.ffmpeg.org/ffmpeg/trunk