Fixes case where the ARM integrated assembler for class InstARM32Mov
did not revert to using the stand-alone assembler (method emit) to
generate the corresponding assembly instructions(s).

This fixes last known problem with method emitIAS (other than
reverting to the stand-alone assembler if not implemented). Removes
use of workaround flag "-unsafe-ias".

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1448783004 .