enabling Hyper-V SMP for ARM64

[schakrabarti_microsoft.com]

Summary

vmbus_synic_setup(), is invoked via vmbus_intrhook -> vmbus_doattach -> smp_rendezvous. On !EARLY_AP_STARTUP (e.g., aarch64), SMP isn't functional in intrhooks and smp_rendezvous() will just call vmbus_synic_setup() on the boot AP. There's nothing that will initialize the pcpu data on every other AP.
To fix it we need to use SI_SUB_SMP for vmbus_doattach().

[kevans]

sys/dev/hyperv/vmbus/vmbus.c

1534

This code should just go away entirely, there is no !EARLY_AP_STARTUP platform where it will ever call vmbus_doattach here.

1586

There's a reference to cold down here in the sysinit that should also go away, we should just always call vmbus_doattach() if we're doing the sysinit -- it *has* to be positioned at a point where "thread scheduling is possible".