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
it's value both meant that interrupts could be assigned and that
there was more than one CPU.

PR: 219882
Reported by: ota@j.email.ne.jp
MFC after: 3 days