x86: move do_nmi(), stop_nmi() and restart_nmi() to traps_64.c

traps_32.c already holds these functions so do the same for traps_64.c

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: hpa@zytor.com
Cc: mingo@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>