gcc port of I422ToARGB_AVX2 from Visual C.  Uses Macros for read of I422 and conversion from YUV to RGB.  Shares constants from I422ToBGRA structure.
BUG=269
TESTED=nacl builds.
R=brucedawson@google.com, tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/27279004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1181 16f28f9a-4ce2-e073-06de-1de4eb20be90