Some timers initialized by the Cobalt core may have a valid reason to
live on CPUs excluded from the real-time set. A typical example would
be the host timer from the scheduler slot, which relays ticks to the
regular kernel. Other core timers are just better dealt with when
created and left passive on those CPUs.

Mark all core timers specifically, and exclude them from the LART
detection code in __xntimer_init(). This fixes a spurious Cobalt debug
assertion seen on SMP at boot, when the real-time CPU set is
restricted to a subset of the online CPU set.