Add ExcludeFromStubs attribute (nanoframework#220)

***NO_CI***

(cherry picked from commit d291a82)

Author: CoryCharlton

nanoFramework.CoreLibrary/System/Runtime/CompilerServices/ExcludeFromStubsAttribute.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// When a class, enum, or struct is marked with <see cref="ExcludeFromStubsAttribute"/> it will be excluded from the native stubs generated by .NET nanoFramework metadata processor.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Struct, AllowMultiple = false, Inherited = true)]
+    public class ExcludeFromStubsAttribute: Attribute
+    {
+    }
+}