[java] make Int64 extern (see #4818)

std/java/StdTypes.hx

@@ -24,7 +24,7 @@ package java;
 @:notNull @:runtimeValue @:coreType abstract Int8 from Int {}
 @:notNull @:runtimeValue @:coreType abstract Int16 from Int {}
 @:notNull @:runtimeValue @:coreType abstract Char16 from Int {}
-@:notNull @:runtimeValue @:coreType abstract Int64 from Int from Float
+@:notNull @:runtimeValue @:coreType extern abstract Int64 from Int from Float
 {
 	@:op(A+B) public static function addI(lhs:Int64, rhs:Int):Int64;
 	@:op(A+B) public static function add(lhs:Int64, rhs:Int64):Int64;