Subzero: Use 'override' as appropriate for C++11.

Not necessary for the LLVM 3.5 merge, but nice to have anyway.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3930
R=jfb@chromium.org, jvoung@chromium.org

Review URL: https://codereview.chromium.org/605123002