runtime: remove redundant empty function call from Breakpoint on arm
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.