System.Private.CoreLib.dll IL binary is getting crossgen'd mistakenly.

This causes both the IL ".dll" and the native image "ni.dll" to be the same size in the Shared Framework. This is incorrect, as the IL binary is much smaller.

The fix is to add ".dll" to the excludedLibraries list in the Crossgen util.

Fix #3522

build_projects/shared-build-targets-utils/Utils/Crossgen.cs

@@ -17,7 +17,7 @@ namespace Microsoft.DotNet.Cli.Build
         {
             "mscorlib.dll",
             "mscorlib.ni.dll",
-            "System.Private.CoreLib",
+            "System.Private.CoreLib.dll",
             "System.Private.CoreLib.ni.dll"
         };