Fixes (at least) the obvious problems with sandboxing and the
integrated assembler. This includes:

Added assembly instruction Nop.

Fixed implementation of padWithNop.

Fixed linking to local label to only fire when persistent (i.e.  last
pass of assembly generation).

Removed restriction on single register push/pop, since the ARM
integrated assembler converts it to a corresopnding str/ldr.

Fixed OperandARM32FlexImm to use smallest rotation value, so that
external assemblers and the ARM integrated assembler will agree on
encoding.

Fixed ARM sandboxing requires test in sandboxing.ll

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1511653002 .