ACPI: Implement power button on !x86

ACPI sleep states are only implemented on x86 systems, so having the
ACPI power button attempt to enter "S5" (or other state as configured
via the hw.acpi.power_button_state sysctl) is not useful.

On non-x86 systems, implement the power button with a call to

shutdown_nice(RB_POWEROFF)

to shut down the system.

Reviewed by: Andrew
Tested on: Graviton 2
MFC after: 2 weeks
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D47094