8009391: Synthetic name of serializable lambda methods should not contain negative numbers

Summary: Use hex representation of method signature hashcode to avoid negative numbers
Reviewed-by: jjg

src/share/classes/com/sun/tools/javac/comp/LambdaToMethod.java

@@ -1299,7 +1299,7 @@ public class LambdaToMethod extends TreeTranslator {
             buf.append(owner.name);
             buf.append('$');
             int methTypeHash = methodSig(owner.type).hashCode();
-            buf.append(methTypeHash);
+            buf.append(Integer.toHexString(methTypeHash));
             buf.append('$');
             String temp = buf.toString();
             Integer count = serializableLambdaCounts.get(temp);