x86: enable NMI watchdog on nosmp

if nosmp has been passed as a boot option, but nmi_watchdog=2 has also
been enabled then keep minimal local APIC functionality around to make
the watchdog work.

this allowed me to debug a hard hang that would only occur with a nosmp
bootup.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>