MdeModulePkg: define PCD for DXE memory protection policy
MdeModulePkg: define PCD for DXE memory protection policy

Define a new fixed/patchable PCD that sets the DXE memory protection
policy: its primary use is to define which memory types should have
their executable permissions removed. Combined with the image protection
policy, this can be used to implement a strict W^X policy, i.e.. a policy
where no regions exist that are both executable and writable at the same
time.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>
Reviewed-by: Jiewen Yao <364a90bfebd1f362ebb7b48e4bf8ec010adef203@intel.com>

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