riscv: Use SBI shutdown call to implement RB_POWEROFF

[jrtc27]

Summary

Currently we only call sbi_shutdown in cpu_reset, which means we reach
"Please press any key to reboot." even when RB_POWEROFF is set, and only
once the user presses a key do we then shutdown. Instead, register a
shutdown_final event handler and make an SBI shutdown call if
RB_POWEROFF is set.

Test Plan

On QEMU, poweroff would previously print "Please press any key to reboot." and
only exit QEMU once a key was pressed, but now causes QEMU to exit immediately.

[br]

looks good.
per style(9) you can insert an empty line if the function has no local variables.

[jhb]

FWIW, we've made that style rule optional, and the rest of this file doesn't use a blank line, so I think it is fine as-is.