6968584: Thread should not be Cloneable

Reviewed-by: dholmes

src/share/classes/java/lang/Thread.java

@@ -413,6 +413,18 @@ class Thread implements Runnable {
         tid = nextThreadID();
     }
 
+    /**
+     * Throws CloneNotSupportedException as a Thread can not be meaningfully
+     * cloned. Construct a new Thread instead.
+     *
+     * @throws  CloneNotSupportedException
+     *          always
+     */
+    @Override
+    protected Object clone() throws CloneNotSupportedException {
+        throw new CloneNotSupportedException();
+    }
+
     /**
      * Allocates a new {@code Thread} object. This constructor has the same
      * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}