Add Sxtb/Sxth instructions to ARM integrated assembler. Refactors code to take advantage of these instructions with Uxtb/Uxth. Note. These instructions are used by Subzero, but not Dart. BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334 R=stichnot@chromium.org Review URL: https://codereview.chromium.org/1503273002 .