Add support for target switches in simulator

[Richard Earnshaw]

> HI Guys,
> 
> > Andrew Cagney <ac131313@cygnus.com> writes:
> 
> > I think a better place would be a new file sim/common/sim-run.h to 
> > indicate that this is part of the interface that run.c uses.
> 
> OK - an adjusted patch is attached below.
> 
> > As for converting Arm to use the new framework, I would recommend 
> > creating a change-request :-)
> 
> I think that would be best.  I certainly feel that it is outside the
> scope of this patch, so I hope that continuing to use the old
> interface (ie not sim-options.[ch]) will not stop the patch from being
> accepted.

I've no objection, provided there would be no user-visible changes if we 
migrated to the standard framework once the PR was addressed.

R.