Add EFI Event When Compatibility Mode is Triggered (#832)

## Description

For a description of Memory Protection Compatibility Mode, see:
https://microsoft.github.io/mu/WhatAndWhy/enhancedmemoryprotection/

This PR adds an event which will be signaled when Compatibility Mode is
activated. Platforms can hook into this event to perform custom actions
when this mode is activated.

- [x] Impacts functionality?
- **Functionality** - Does the change ultimately impact how firmware
functions?
- Examples: Add a new library, publish a new PPI, update an algorithm,
...
- [ ] Impacts security?
- **Security** - Does the change have a direct security impact on an
application,
    flow, or firmware?
  - Examples: Crypto algorithm change, buffer overflow fix, parameter
    validation improvement, ...
- [ ] Breaking change?
- **Breaking change** - Will anyone consuming this change experience a
break
    in build or boot behavior?
- Examples: Add a new library class, move a module to a different repo,
call
    a function in a new library class in a pre-existing module, ...
- [ ] Includes tests?
  - **Tests** - Does the change include any explicit test code?
  - Examples: Unit tests, integration tests, robot tests, ...
- [ ] Includes documentation?
- **Documentation** - Does the change contain explicit documentation
additions
    outside direct code modifications (and comments)?
- Examples: Update readme file, add feature readme file, link to
documentation
    on an a separate Web page, ...

## How This Was Tested

Tested by booting Q35 to shell.

## Integration Instructions

N/A

MdeModulePkg/Core/Dxe/DxeMain.inf

@@ -131,6 +131,7 @@
   gMuEventPreExitBootServicesGuid               ## PRODUCES             ## Event    // MU_CHANGE
   gDxeMemoryProtectionSettingsGuid              ## CONSUMES             ## HOB      // MU_CHANGE
   gMemoryProtectionSpecialRegionHobGuid         ## CONSUMES             ## HOB      // MU_CHANGE
+  gCompatibilityModeActivatedEventGuid          ## SOMETIMES_PRODUCES   ## Event    // MU_CHANGE
 
 [Ppis]
   gEfiVectorHandoffInfoPpiGuid                  ## UNDEFINED # HOB

MdeModulePkg/Core/Dxe/Misc/MemoryProtectionSupport.c

@@ -2884,6 +2884,7 @@ ActivateCompatibilityMode (
   DisableNullDetection ();
   UninstallMemoryAttributeProtocol ();
   MapLegacyBiosMemoryRWX ();
+  CoreNotifySignalList (&gCompatibilityModeActivatedEventGuid);
 }
 
 /**

MdeModulePkg/Include/Guid/CompatibilityModeActivatedEvent.h

@@ -0,0 +1,18 @@
+/** @file
+  Event group triggered when Memory Protection Compatibility Mode is activated.
+  Platforms can hook into this event to perform follow up actions.
+  More information on Compatibility Mode: https://microsoft.github.io/mu/WhatAndWhy/enhancedmemoryprotection/
+
+  Copyright (c) Microsoft Corporation.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#ifndef COMPATIBILITY_MODE_ACTIVATED_EVENT_H_
+#define COMPATIBILITY_MODE_ACTIVATED_EVENT_H_
+
+#define COMPATIBILITY_MODE_ACTIVATED_EVENT_GUID \
+   { 0x209BA820, 0xCE32, 0x49E1, { 0x95, 0xAE, 0x39, 0x43, 0x3D, 0xEF, 0xD5, 0xEE } }
+
+extern EFI_GUID  gCompatibilityModeActivatedEventGuid;
+
+#endif

MdeModulePkg/MdeModulePkg.dec

@@ -510,6 +510,11 @@
   # Include/Guid/MemoryProtectionSpecialRegionGuid.h
   gMemoryProtectionSpecialRegionHobGuid = { 0xBFCC1325, 0x3BFE, 0x469A, { 0x9A, 0xAC, 0x0C, 0x99, 0x0E, 0x4E, 0xC9, 0xF1 } }
 
+  # MU_CHANGE
+  # This GUID is used to trigger the event group when the compatibility mode is activated.
+  # Include/Guid/CompatibilityModeActivatedEvent.h
+  gCompatibilityModeActivatedEventGuid = { 0x209BA820, 0xCE32, 0x49E1, { 0x95, 0xAE, 0x39, 0x43, 0x3D, 0xEF, 0xD5, 0xEE } }
+
 ## MSCHANGE END