Extends the ARM32 assembler to be able to generate a trivial function
footprint using the -filetype=iasm option.

Also does a couple of cleanups:

1) Move UnimplementedError macro to common location so that it can be
used by everyone.

2) Add a GlobalContext argument to the assembler, so that it can
look at flags etc.

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

Review URL: https://codereview.chromium.org/1397933002 .