x86: move all asm/pgtable constants into one place

32 and 64-bit use the same flags for pagetable entries, so make them all common.

[ mingo@elte.hu: fixes ]

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>