arm64: add a spin-table implementation for Apple Silicon

[kevans]

Summary

The M1 has no EL3, so we're limited to a spin-table implementation if we
want to eventually use bhyve on it. Implement spin-table now, but note
that we still prefer PSCI where possible.

[kevans]

This seems to work, at least, in the sense that we proceed with boot so the other cores must have made it into init_secondary(). We're not quite at AP release, though.

[kevans]

I can confirm this is doing the right thing, at least on this hardware; I now get to mountroot past AP release after fixing some other bits with the interrupt controller.

[kevans]

With rG866beaa0aa95 this has successfully booted a Raspberry Pi 4 both with our armstub8* that does PSCI and with the upstream stock armstub8*, and we use this actively on M1 WIP as well.

sys/arm64/arm64/mp_machdep.c

524

Will apply this pre-commit

[mmel]

sys/arm64/arm64/mp_machdep.c

527

I think we should do a full(system) DSB before the SEV (although it seems a bit paranoid, I know). Rest looks OK for me.

[kevans]

Toss in a barrier prior to wakeup; perhaps paranoid, but it seems like a
reasonable concern

[mmel]

Thanks .