Propertly handle the layout of generic valuetypes. This is a fix similar to 2a899eb (case 930358)

Author: joncham

mono/metadata/class.c

@@ -1602,7 +1602,7 @@ mono_class_layout_fields (MonoClass *class)
 	 /* corlib is missing [StructLayout] directives in many places */
 	if (layout == TYPE_ATTRIBUTE_AUTO_LAYOUT) {
 		if (class->image != mono_defaults.corlib &&
-			class->byval_arg.type != MONO_TYPE_VALUETYPE)
+			!class->valuetype)
 			gc_aware_layout = TRUE;
 		/* from System.dll, used in metadata/process.h */
 		if (strcmp (class->name, "ProcessStartInfo") == 0)