CL 18964 included an extra patch (sorry, my first experience of
git-codereview) which defined the conventional breakpoint instruction
used by Plan 9 on arm, but also introduced a benign but unneeded
call to runtime.emptyfunc.  This CL removes the redundant call again.

This completes the series of CLs which add support for Plan 9 on arm.

Change-Id: Id293cfd40557c9d79b4b6cb164ed7ed49295b178
Reviewed-on: https://go-review.googlesource.com/19010Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>