sched: Add WAKEUP_PREEMPTION feature flag, on by default

As per the recent discussion with Mike and Linus, make it easier to
test with/without this feature. No change in default behavior.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/n/tip-izoxq4haeg4mTognnDbwcevt@git.kernel.org