gcc version of I422ToBGRA_AVX2.  Original copied from https://webrtc-codereview.appspot.com/28729004/ and compatible with, but unrelated to windows version.
BUG=269
TESTED=untested
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/29849004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1131 16f28f9a-4ce2-e073-06de-1de4eb20be90