MFC r319942 (jhb):

Don't try to assign interrupts to a CPU on single-CPU systems.

All interrupts are routed to the sole CPU in that case implicitly.
This is a regression in EARLY_AP_STARTUP. Previously the 'assign_cpu'
variable was only set when a multi-CPU system finished booting, so
its value both meant that interrupts could be assigned and that
there was more than one CPU.

PR: 219882
Approved by: re (kib)
Sponsored by: The FreeBSD Foundation