Prevent symbols defined in assembly sources from being exported in
libraries which include them by marking them hidden, as they are
implementation details.
BUG=webrtc:4183
R=andrew@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/36759004
Patch from Richard Coles <torne@chromium.org>.
Cr-Commit-Position: refs/heads/master@{#8658}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8658 4adac7df-926f-26a2-2b94-8c16560cd09d