Previously, the code assumed that the emit() method of all ARM
instructions emitted a single instruction. This is false. Instructions
like PUSH and POP may generate multiple instructions.

This is only a problem when the hybrid ARM assembler reverts back to
using the stand-alone assembler to generate instructions the
integrated assembler can't handle.

The fix is to add infrastructure to allow ARM instructions to
communicate to the assembler, the number of instructions they
generate, so that the correct-sized filler is added to the assembly
buffer.

This fixes all cross-test failures for (pc-relative) branches, except
one.

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

Review URL: https://codereview.chromium.org/1426513004 .