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: NDavid Howells <dhowells@redhat.com>