powerpc: Fix SMP time initialization problem

We were getting the last_jiffy per-cpu variable set ahead of the current
timebase in smp_space_timers on SMP machines.  This caused the loop in
timer_interrupt to loop virtually forever, since tb_ticks_since assumes
that it will never be called with the timebase behind the last_jiffy
value.

Signed-off-by: Paul Mackerras <paulus@samba.org>