don't take an arg, but swi_generic() is special in order to avoid one
whole conditional branch in the old SWI dispatch code. The new SWI
dispatch code passed it a garbage arg. Bypass swi_generic() and call
swi_dispatcher() directly, like the corresponding alpha code has always
done.
The panic was rare because because it only occurred if more than one
of the {sio,cy,rc} drivers was configured and one was active, and the
cy driver doesn't even compile.
d511196c02