Rework handling of thread sleeps before timers are working.

[jhb]

Summary

Rework handling of thread sleeps before timers are working.

Previously, calls to *sleep() and cv_*wait*() immediately returned during
early boot. Instead, permit threads that request a sleep without a
timeout to sleep as wakeup() works during early boot. Sleeps with
timeouts are harder to emulate without working timers, so just punt and
panic explicitly if any thread tries to use those before timers are
working. Any threads that depend on timeouts should either wait until
SI_SUB_KICK_SCHEDULER to start or they should use DELAY() until timers
are available.

Until APs are started earlier this should be a no-op as other kthreads
shouldn't get a chance to start running until after timers are working
regardless of when they were created.

Test Plan

• boot kernels both with just this change but also with the rest of the early AP startup patches with EARLY_AP_STARTUP applied

[kib]

What about lockmgr, it also uses sleepq_timedwait() ?

Might be, the cold panic should be moved to sleepq_timedwait() ?

[jhb]

In D5724#122295, @kib wrote:

What about lockmgr, it also uses sleepq_timedwait() ?

Might be, the cold panic should be moved to sleepq_timedwait() ?

Mmmm, yes.

[jhb]

• Move the panic for timed sleeps into sleepq_set_timeout_sbt().