NV12ToARGB for linux appears to have a bug in the assembly so this disables the asm for posix.  It still works on Windows.  Unittests updated to show the amount of pixel difference.
BUG=55
TEST=out/Release/libyuv_unittest --gtest_filter=*
Review URL: https://webrtc-codereview.appspot.com/675008

git-svn-id: http://libyuv.googlecode.com/svn/trunk@310 16f28f9a-4ce2-e073-06de-1de4eb20be90