x86: add pgd_large() on 64-bit, for consistency

In order to have it at all levels, add pgd_large() which only
returns 0.

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>