This fixes linking shared libavcodec, since the linker doesn't allow
text relocations in shared libraries under Darwin/x86_64.
Based on a patch by Art Clarke (aclarke xuggle com)
Originally committed as revision 17197 to svn://svn.ffmpeg.org/ffmpeg/trunk
It contains optimizations that are not specific to i386 and
libavutil uses this naming scheme already.
Originally committed as revision 16270 to svn://svn.ffmpeg.org/ffmpeg/trunk