During device attachment, all interrupt sources will bind to the BSP,
as it is the only processor online. This means interrupts must be
redistributed later, during SI_SUB_SMP.

For the EARLY_AP_STARTUP case, this is no longer true. SI_SUB_SMP will
execute much earlier, meaning APs will be online and available before
devices begin attachment, and there will therefore be nothing to
shuffle.

All PIC-conforming interrupt controllers will handle this early
distribution properly, except for RISC-V's PLIC. Make the necessary
tweak to the PLIC driver.

While here, convert irq_assign_cpu from boolean_t to a bool.