clean up the preemption check to not use unnecessary 64-bit
variables. This improves code size:

   text    data     bss     dec     hex filename
  44227    3326      36   47589    b9e5 sched.o.before
  44201    3326      36   47563    b9cb sched.o.after
Signed-off-by: NIngo Molnar <mingo@elte.hu>