ArmPkg/ArmMmuLib ARM: implement memory permission control routines

Now that we have the prerequisite functionality available in ArmMmuLib,
wire it up into ArmSetMemoryRegionNoExec, ArmClearMemoryRegionNoExec,
ArmSetMemoryRegionReadOnly and ArmClearMemoryRegionReadOnly. This is
used by the non-executable stack feature that is configured by DxeIpl.

NOTE: The current implementation will not combine RO and XP attributes,
      i.e., setting/clearing a region no-exec will unconditionally
      clear the read-only attribute, and vice versa. Currently, we
      only use ArmSetMemoryRegionNoExec(), so for now, we should be
      able to live with this.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>