[std/hl] move some position for customStack and set_stack (#12217)

Author: yuxiaomao

std/hl/_std/haxe/Exception.hx

@@ -13,6 +13,7 @@ class Exception {
 	@:noCompletion @:ifFeature("haxe.Exception.get_stack") var __skipStack:Int = 0;
 	@:noCompletion var __nativeException:Any;
 	@:noCompletion var __previousException:Null<Exception>;
+	@:noCompletion var __customStack:Null<String>;
 
 	static function caught(value:Any):Exception {
 		if(Std.isOfType(value, Exception)) {
@@ -91,10 +92,7 @@ class Exception {
 	}
 
 	function set_stack(stack:CallStack) {
-		__exceptionStack = stack;
 		__customStack = CallStack.toString(stack);
-		return stack;
+		return __exceptionStack = stack;
 	}
-
-	@:noCompletion var __customStack:Null<String>;
-}
+}