The cmpxchg instruction already sets ZF for comparing the return value
vs the expected value. So there is no need to compare eq again.

Lots of pexes-in-the-wild have this pattern. Some compare against
a constant, some compare against a variable.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3882
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/413903002