Redo r12838, this time using svn copy to create h264_i386.h from cabac.h.

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 12846 to svn://svn.ffmpeg.org/ffmpeg/trunk