New target libyuv_untrusted compiles libyuv for PNaCl.

BUG=276739
R=fbarchard@google.com

Review URL: https://webrtc-codereview.appspot.com/6299004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@969 16f28f9a-4ce2-e073-06de-1de4eb20be90