ArmVirtPkg: clean up PcdSetNxForStack setting (applies to ArmVirtQemu only)

ArmVirtPkg: clean up PcdSetNxForStack setting (applies to ArmVirtQemu only)

According to the

  PCDs not used by modules or in conditional directives

sections of all the build reports for

  {AARCH64,ARM} x
  {QemuKernel,Xen} x
  {DEBUG,NOOPT,RELEASE} x
  {feat-0,feat-1}

(24 builds in total), the PCD is not used in any of those builds.

Move the setting from "ArmVirt.dsc.inc" to "ArmVirtQemu.dsc", to reflect
reality.

We originally moved the PCD setting in the opposite direction in commit
8aab575c26e9 ("ArmVirtPkg: enable non-executable DXE stack for all
platforms", 2017-03-07), generalizing it. However, as the comment itself
states, and according to all 36 ArmVirt build reports:

  {AARCH64,ARM} x
  {Qemu,QemuKernel,Xen} x
  {DEBUG,NOOPT,RELEASE} x
  {feat-0,feat-1}

the PCD is only consumed by "MdeModulePkg/Core/DxeIplPeim/DxeIpl.inf", and
that module is only included in the ArmVirtQemu platform.

("feat-1" stands for "-D HTTP_BOOT_ENABLE -D NETWORK_IP6_ENABLE -D
SECURE_BOOT_ENABLE -D TTY_TERMINAL", while "feat-0" stands for "".)

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Julien Grall <julien.grall@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>