ArmVirtPkg AARCH64: enable NX memory protection for all platforms

This sets the recently introduced PCD PcdDxeNxMemoryProtectionPolicy to
a value that protects all memory regions except code regions against
inadvertent execution.

Note that this does not [yet] protect EfiLoaderData regions, due to
compatibility issues with shim and GRUB.

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