Fixes emit() methods for load/store to specify the element size (affects
alignment issues).

Also adds assembler methods to generate the corresponding binary forms,
and updates emitIAS() to call these assembler methods.

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

Review URL: https://codereview.chromium.org/1663053008 .