ia64/pv_ops/xen: define xen specific gate page.

define xen specific gate page.
At this phase bits in the gate page is same to native.
At the next phase, it will be paravirtualized.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>