This fixes the scheduler's sync-wakeup code to be consistent on UP as
well.

Right now there's a behavioral difference between an UP kernel and an
SMP kernel running on a UP box: sync wakeups (which are only activated
on SMP) can cause a wakeup of a higher prio task, without preemption.
On UP kernels this does not happen.  This difference in wakeup behavior
is bad.

This patch activates sync wakeups on UP as well - in the cases sync
wakeups are done the waker knows that it will schedule away soon, so
this 'delay preemption' decision is correct on UP as well.