Fix AtomicObject cpp error conditional (#11674)

std/haxe/atomic/AtomicObject.hx

@@ -4,7 +4,7 @@ package haxe.atomic;
 #error "This target does not support atomic operations."
 #end
 
-#if (js || hxcpp)
+#if (js || cpp)
 #error "JavaScript and Hxcpp do not support AtomicObject"
 #end