Subzero. Adds ldrex, strex, and dmb support (ARM32)

These instructions are used to load/store data atomically, and to
notify the processor about a data memory barrier. They are used for
implementing the llvm.nacl.atomic.* lowerings.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1378303003 .