Subzero: Support non sequentially consistent memory orderings for atomic ops.

The actual code lowering is unchanged, but the validation is made less strict to allow the additional orderings.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4029
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/1017453007
6 files changed