Initialize 'ticks' earlier in boot after 'hz' is set.

This avoids the time-warp after kthreads have started running and the
required fixup to td_slptick and td_blktick in the EARLY_AP_STARTUP
case. Now, 'ticks' is initialized before any kthreads are created or
any context switches are performed.

Tested by: gavin
MFC after: 2 weeks
Sponsored by: Netflix