ArmPkg/AArch64Mmu: use architecturally correct definitions for XN/UXN

ArmPkg/AArch64Mmu: use architecturally correct definitions for XN/UXN

The non-privileged execute never (UXN) page table bit defined for the
EL1&0 translation regime and the execute never (XN) bit defined for the
EL2 and EL3 translation regimes happen to share the same bit position,
but they are in fact defined distinctly by the architecture. So define
both bits explicitly, and add comments in places where we take advantage
of the fact that they share the same bit position.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18585 6f19259b-4bc3-4df7-8a09-765794883524