Add attribute that forces alignment of stack to functions that need it.

Add attribute that forces alignment of stack to functions that need it.
Necessary for systems that don't align by default to 16 bytes, required by some
SSE instructions.
Requires GCC >= 4.2.
Based on patch by Gaël Chardon.

Originally committed as revision 10106 to svn://svn.ffmpeg.org/ffmpeg/trunk