Simplify cpu_switchto() such that it doesn't need to know the number of register windows provided by the hardware; this number is not readily available on sun4v.
Simplify cpu_switchto() such that it doesn't need to know the number of
register windows provided by the hardware; this number is not readily
available on sun4v.

This removes the optimization that skips flushing register windows if
a process exits.  We can add that back later if it turns out to make a
significant impact.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars