acpi: Implement s2idle loop

[obiwac]

Summary

When entering s2idle, we might still receive spurious SCIs from devices we can't mask GPEs from (well, we can mask them but that would mean masking out wake GPEs too). To solve this, put everything in a loop where every CPU except CPU0 is idled, and, when CPU0 is woken up, check if we are meant to break out of this loop and wake all other CPUs or if we should immediately go back to sleep.