initclocks: don't reinitialize ticks, jiffies at runtime

initclocks: don't reinitialize ticks, jiffies at runtime

Various drivers use ticks/jiffies before initclocks().  It isn't
generally safe to reinitialize them at runtime.  Hoist the conditional
definition of HZ from param.c into sys/kernel.h so we can see it from
kern_clock.c and statically initialize ticks/jiffies to the desired
offset.

With this change, timeouts scheduled before initclocks() do not all
fire immediately during the first softclock().

With input from kettenis@.

Link: https://marc.info/?l=openbsd-tech&m=167753870803378&w=2