x86: require family >= 6 if we are using P6 NOPs

The P6 family of NOPs are only available on family >= 6 or above, so
enforce that in the boot code.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>