x86: make nmi_shootdown_cpus() available on !SMP and !X86_LOCAL_APIC

Impact: widen nmi_shootdown_cpus() availability

The X86_LOCAL_APIC #ifdef was for kdump. For !SMP, the function simply
does nothing.

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