ArmVirtPkg: enable non-executable DXE stack for all platforms

Now that ARM has grown support for managing memory permissions in
ArmMmuLib, we can enable the non-executable DXE stack for all virt
platforms. Note that this includes the AARCH64 Xen platform as well.

Note that this is not [entirely] redundant: the non-executable stack
is configured before DxeCore is invoked. The image and memory protection
features configured during DXE only take affect when the CPU arch
protocol implementation is registered.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>