8321470: ThreadLocal.nextHashCode can be static final

Reviewed-by: shade, jpai

Author: bokken

src/java.base/share/classes/java/lang/ThreadLocal.java

@@ -97,7 +97,7 @@ public class ThreadLocal<T> {
      * The next hash code to be given out. Updated atomically. Starts at
      * zero.
      */
-    private static AtomicInteger nextHashCode =
+    private static final AtomicInteger nextHashCode =
         new AtomicInteger();
 
     /**