Remove AltiVec vector declaration compiler compatibility macros.

The original problem was that FSF and Apple gcc used a different syntax
for vector declarations, i.e. {} vs. (). Nowadays Apple gcc versions support
the standard {} syntax and versions that support {} are available on all
relevant Mac OS X versions. Thus the greater compatibility is no longer
worth cluttering the code with macros.

Originally committed as revision 14366 to svn://svn.ffmpeg.org/ffmpeg/trunk