x86, hotplug: Wake up CPU0 via NMI instead of INIT, SIPI, SIPI

Instead of waiting for STARTUP after INITs, BSP will execute the BIOS boot-strap
code which is not a desired behavior for waking up BSP. To avoid the boot-strap
code, wake up CPU0 by NMI instead.

This works to wake up soft offlined CPU0 only. If CPU0 is hard offlined (i.e.
physically hot removed and then hot added), NMI won't wake it up. We'll change
this code in the future to wake up hard offlined CPU0 if real platform and
request are available.

AP is still waken up as before by INIT, SIPI, SIPI sequence.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1352896613-25957-1-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/include/asm/cpu.h

@@ -28,6 +28,7 @@ struct x86_cpu {
 #ifdef CONFIG_HOTPLUG_CPU
 extern int arch_register_cpu(int num);
 extern void arch_unregister_cpu(int);
+extern void __cpuinit start_cpu0(void);
 #endif
 
 DECLARE_PER_CPU(int, cpu_state);