x86: add common mm/pgtable.c

Add a common arch/x86/mm/pgtable.c file for common pagetable functions.

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