x86: move nmi_shootdown_cpus() to reboot.c

Impact: make nmi_shootdown_cpus() available to the rest of the x86 platform

Now nmi_shootdown_cpus() is ready to be used by non-kdump code also.
Move it to reboot.c.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>