.azurepipelines: Update Core CI build matrix to include platforms

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2570

Add ArmVirtPkg to Core CI matrix
Add EmulatorPkg to Core CI matrix
Add OvmfPkg to Core CI matrix

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

Reviewed-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>

.azurepipelines/templates/pr-gate-build-job.yml

@@ -44,6 +44,11 @@ jobs:
       TARGET_SECURITY:
         Build.Pkgs: 'SecurityPkg'
         Build.Targets: 'DEBUG,RELEASE,NO-TARGET'
+      TARGET_PLATFORMS:
+        # For Platforms only check code. Leave it to Platform CI
+        # to build them.
+        Build.Pkgs: 'ArmVirtPkg,EmulatorPkg,OvmfPkg'
+        Build.Targets: 'NO-TARGET'
 
   workspace:
     clean: all