x86, NMI watchdog: add support to enable and disable IOAPIC NMI

Impact: change/improve the way /proc/sys/kernel/nmi_watchdog works

This patch adds support to enable/disable IOAPIC NMI watchdog in runtime via
procfs.

Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Cc: "Maciej W. Rozycki" <macro@linux-mips.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>