[mono] Prevent direct MonoClass field access

Fixes mono/mono#15544

### After adding configure flag
<img width="874" alt="Screen Shot 2020-01-30 at 10 54 25 AM" src="https://user-images.githubusercontent.com/16830051/73466184-81f4c180-434f-11ea-87f0-0863c32b1251.png">

### After using m_class accessors
<img width="865" alt="Screen Shot 2020-01-30 at 10 55 04 AM" src="https://user-images.githubusercontent.com/16830051/73466229-93d66480-434f-11ea-8914-cef5571a0528.png">

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src/mono/mono/mini/mini-wasm-debugger.c

@@ -655,11 +655,11 @@ static gboolean describe_value(MonoType * type, gpointer addr)
 			} else {
 				GString *class_name;
 				class_name = g_string_new ("");
-				if (*(obj->vtable->klass->name_space)) {
-					g_string_append (class_name, obj->vtable->klass->name_space);
+				if (*(m_class_get_name_space (obj->vtable->klass))) {
+					g_string_append (class_name, m_class_get_name_space (obj->vtable->klass));
 					g_string_append_c (class_name, '.');
 				}
-				g_string_append (class_name, obj->vtable->klass->name);
+				g_string_append (class_name, m_class_get_name (obj->vtable->klass));
 				if (m_class_get_byval_arg (obj->vtable->klass)->type == MONO_TYPE_SZARRAY || m_class_get_byval_arg (obj->vtable->klass)->type == MONO_TYPE_ARRAY)
 					mono_wasm_add_array_var (class_name->str, get_object_id(obj));
 				else
@@ -821,11 +821,11 @@ describe_this (MonoStackFrameInfo *info, MonoContext *ctx, gpointer ud)
 		mono_wasm_add_properties_var("this");
 		GString *class_name;
 		class_name = g_string_new ("");
-		if (*(obj->vtable->klass->name_space)) {
-			g_string_append (class_name, obj->vtable->klass->name_space);
+		if (*(m_class_get_name_space (obj->vtable->klass))) {
+			g_string_append (class_name, m_class_get_name_space (obj->vtable->klass));
 			g_string_append_c (class_name, '.');
 		}
-		g_string_append (class_name, obj->vtable->klass->name);
+		g_string_append (class_name, m_class_get_name (obj->vtable->klass));
 		mono_wasm_add_obj_var (class_name->str, get_object_id(obj));
 		g_string_free(class_name, FALSE);
 	}