Add a macro that clears the want_resched flag that need_resched sets.

Add a macro that clears the want_resched flag that need_resched sets.
Right now when mi_switch picks up the same proc, we didn't clear the
flag which would mean that every time we service an AST we would attempt
a context switch. For some architectures, amd64 being probably the
most extreme, that meant attempting to context switch for every
trap and interrupt.

Now we clear_resched explicitly after every context switch, even if it
didn't do anything. Which also allows us to remove some more code
in cpu_switchto (not done yet).

miod@ ok