KGDB: Notify GDB of machine halt, reboot or power off

Notify GDB of the machine halting, rebooting or powering off by sending it an
exited command (remote protocol command 'W').  This is done by calling:

	void gdbstub_exit(int status)

from the arch's machine_{halt,restart,power_off}() functions with an
appropriate exit status to be reported to GDB.

Signed-off-by: David Howells <dhowells@redhat.com>