Fix uapaot build issues for System.Runtime

baseline readonly struct issue

Author: weshaggard

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

@@ -6547,7 +6547,9 @@ public RuntimeCompatibilityAttribute() { }
     }
     public static partial class RuntimeFeature
     {
+#if FEATURE_DEFAULT_INTERFACES
         public const string DefaultImplementationsOfInterfaces = "DefaultImplementationsOfInterfaces";
+#endif
         public static bool IsSupported(string feature) { throw null; }
     }
     public static partial class RuntimeHelpers

src/System.Runtime/src/ApiCompatBaseline.uapaot.txt

@@ -1 +1,2 @@
-TypeCannotChangeClassification : Type 'System.TypedReference' is a 'ref struct' in the implementation but is a 'struct' in the contract.
+TypeCannotChangeClassification : Type 'System.TypedReference' is a 'ref struct' in the implementation but is a 'struct' in the contract.
+TypeCannotChangeClassification : Type 'System.Runtime.CompilerServices.YieldAwaitable.YieldAwaiter' is marked as readonly in the contract so it must also be marked readonly in the implementation.