Respect the kern.smp.disabled tunable. When we're scanning the MADT in
ia64_probe_sapics(), we also create PCPU structures for any Local SAPICs
we encounter. When SMP is disabled, this leaves us with partially setup
PCPU structures, which typically results in panics when we're iterating
over CPUs. When SMP is disabled, we now prevent the creation of the
PCPU structures.