MFC: r234247

Merge from x86:

r233961:

Fix interrupt load balancing regression, introduced in revision
222813, that left all un-pinned interrupts assigned to CPU 0.
In intr_shuffle_irqs(), remove CPU_SETOF() call that initialized
the "intr_cpus" cpuset to only contain CPU0.

This initialization is too late and nullifies the results of calls
to the intr_add_cpu() that occur much earlier in the boot process.

r234074 (partial):

The BSP is not added to the mask of valid target CPUs for interrupts.
Fix this by adding the BSP as an interrupt target directly in

r234105:

Fix !SMP build after r234074.