x86: separate cmpxchg8b checking from PAE checking

.. allowing the former to be use in non-PAE kernels, too.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>