Port x264 deblocking code to libavcodec.

This includes SSE2 luma deblocking code and both MMXEXT and SSE2 luma
intra deblocking code for H.264 decoding. This assembly is available
under --enable-gpl and speeds decoding of Cathedral by 7%.

Originally committed as revision 16239 to svn://svn.ffmpeg.org/ffmpeg/trunk