arm64: add a spin-table implementation for Apple Silicon

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.

Reviewed by: mmel

(cherry picked from commit 753a23ac152c5024de34f9eb02a2fdccf2881085)