Make JsonSerializer work when IsDynamicCodeSupported is false. (#54027)

Use the non-Emit codepath if IsDynamicCodeSupported is false.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.cs

@@ -559,7 +559,12 @@ internal MemberAccessor MemberAccessorStrategy
             {
                 if (_memberAccessorStrategy == null)
                 {
-#if NETFRAMEWORK || NETCOREAPP
+#if NETCOREAPP
+                    // if dynamic code isn't supported, fallback to reflection
+                    _memberAccessorStrategy = RuntimeFeature.IsDynamicCodeSupported ?
+                        new ReflectionEmitMemberAccessor() :
+                        new ReflectionMemberAccessor();
+#elif NETFRAMEWORK
                     _memberAccessorStrategy = new ReflectionEmitMemberAccessor();
 #else
                     _memberAccessorStrategy = new ReflectionMemberAccessor();