Add the DisableRuntimeMarshallingAttribute to the build.

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -711,6 +711,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\DefaultDependencyAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\DependencyAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\DisablePrivateReflectionAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\DisableRuntimeMarshallingAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\DiscardableAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\EnumeratorCancellationAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ExtensionAttribute.cs" />

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/DisableRuntimeMarshallingAttribute.cs

@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+/*=============================================================================
+**
+**
+**
+** Purpose: Mark up the program to indicate various legacy or new opt-in behaviors.
+**
+**
+=============================================================================*/
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Assembly, Inherited = false, AllowMultiple = false)]
+    public sealed class DisableRuntimeMarshallingAttribute : Attribute
+    {
+    }
+}

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -12972,6 +12972,11 @@ public sealed partial class DisablePrivateReflectionAttribute : System.Attribute
     {
         public DisablePrivateReflectionAttribute() { }
     }
+    [System.AttributeUsageAttribute(System.AttributeTargets.Assembly, Inherited = false, AllowMultiple = false)]
+    public sealed class DisableRuntimeMarshallingAttribute : Attribute
+    {
+        public DisableRuntimeMarshallingAttribute() { }
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.All)]
     public partial class DiscardableAttribute : System.Attribute
     {