ArmVirtPkg/HighMemDxe: use CPU arch protocol to apply memprotect policy

Instead of invoking gDS->SetMemorySpaceAttributes to set the EFI_MEMORY_XP
attribute on newly added regions, which is guaranteed to fail if the same
attribute was not declared as a capability of the region when it as added,
invoke the CPU arch protocol directly to set the EFI_MEMORY_XP attribute
if our memory protection policy demands it.

Reported-by: Michael Zimmermann <sigmaepsilon92@gmail.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>