ACPI: Implement power button on !x86

[cperciva]

Summary

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.

Sponsored by: Amazon
Tested on: Graviton 2
MFC after: 2 weeks

[cperciva]

I could add a sysctl to disable this if anyone thinks that making the power button not power-off the system would be a useful option.

[andrew]

It looks like this would fix https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273756

[cperciva]

In D47094#1073271, @andrew wrote:

It looks like this would fix https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273756

Maybe? There are a number of different ways to tell a system to turn off, so I don't know for certain whether the ACPI shutdown event from QEMU is this particular one.