x86/xen: mark xen_pv_play_dead() as __noreturn

Mark xen_pv_play_dead() and related to that xen_cpu_bringup_again()
as "__noreturn".

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20221125063248.30256-3-jgross@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>