Fix Subzero binary instruction to allow optional flags argument. This CL fixes pnacl-sz to handle smoothnacl.pexe. BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4364 R=jpp@chromium.org, stichnot@chromium.org Review URL: https://codereview.chromium.org/1831043002 .