6745216: missing 4 chraset aliases in sun.nio.cs package

Summary: added "834" into x-IBM834's aliase list.
Reviewed-by: alanb

src/share/classes/sun/nio/cs/ext/ExtendedCharsets.java

@@ -770,6 +770,7 @@ public class ExtendedCharsets
                 new String[] {
                     "cp834",
                     "ibm834",
+                    "834",
                     "ibm-834"
         });