Make Throwable properties message and cause open.

#KT-5587 Fixed

core/builtins/native/kotlin/Throwable.kt

@@ -22,7 +22,7 @@ package kotlin
  * @param message the detail message string.
  * @param cause the cause of this throwable.
  */
-public open class Throwable(val message: String?, val cause: Throwable?) {
+public open class Throwable(open val message: String?, open val cause: Throwable?) {
     constructor(message: String?) : this(message, null)
 
     constructor(cause: Throwable?) : this(cause?.toString(), cause)