Use clang front end ifdef for AVX2 instead of llvm backend.
BUG=269
TESTED=clang++ -c source/row_posix.cc -I include/
R=ryanpetrie@google.com

Review URL: https://webrtc-codereview.appspot.com/2266004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@796 16f28f9a-4ce2-e073-06de-1de4eb20be90
3 files changed