x86: export __supported_pte_mask

export __supported_pte_mask variable as GPL symbol.
lguest is a user of it.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>