When interrupt vectors are first allocated, they get assigned to
CPU #0; at SI_SUB_SMP / SI_ORDER_SECOND (aka once we have multiple
CPUs), the intr_irq_shuffle SYSINIT clears their CPU sets with the
effect of forcing them to be assigned to new CPUs later.

In case where interrupt vectors were allocated *but not yet bound*
this code did not run, with the effect that those interrupts would
remain pinned to CPU #0 forever. This affected the ena(4) driver,
which allocates interrupts for I/O when the device is attached but
doesn't set them up until the interface is brought up much later in
the boot process (and, crucially, long after intr_irq_shuffle runs).

Adjust intr_irq_shuffle to clear the CPU set for an interrupt source
even if it currently has no handlers, so that it will be properly
assigned to a CPU when it is used later.

MFC after: 1 month
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