use macro Use DECLARE_ALIGNED_16 to align stack-allocated variables

instead of compiler-dependent  __attribute__((aligned(16)))
Origiginal thread:
Date: May 17, 2007 12:30 AM
Subject: [PATCH] Use DECLARE_ALIGNED_16 in libavcodec/ppc/

Originally committed as revision 9047 to svn://svn.ffmpeg.org/ffmpeg/trunk