x86: unify pgtable accessors which use

Make users of supported_pte_mask common.  This has the side-effect of
introducing the variable for 32-bit non-PAE, but I think its a pretty
small cost to simplify the code.

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