acpi: Make taskqueue only run on BSP

This is needed because when we add the suspend-to-idle loop, we only
want to break the BSP out of idle to process the taskqueue while keeping
all the others idled.

Currently assuming BSP to be CPU0, which may not always be the case on
non-x86.

Reviewed by: kib
Approved by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54406