xen: check EFER for NX before setting up GDT mapping

xen: check EFER for NX before setting up GDT mapping

commit b75fe4e5b869f8dbebd36df64a7fcda0c5b318ed upstream.

x86-64 assumes NX is available by default, so we need to
explicitly check for it before using NX.  Some first-generation
Intel x86-64 processors didn't support NX, and even recent systems
allow it to be disabled in BIOS.

[ Impact: prevent Xen crash on NX-less 64-bit machines ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>