Mark stack as not executable.  Although its tempting to generate code and execute it as an optimization, security says this is a bad thing.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/989005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@518 16f28f9a-4ce2-e073-06de-1de4eb20be90