Userland: Fix unnecessary heap allocation of singleton objects

In order to avoid having multiple instances, we were keeping a pointer
to these singleton objects and only allocating them when it was null.

We have `__cxa_guard_{acquire,release}` in the userland, so there's no
need to do this dance, as the compiler will ensure that the constructors
are only called once.

Userland/Libraries/LibGUI/DisplayLink.cpp

@@ -31,10 +31,8 @@ private:
 
 static HashMap<i32, RefPtr<DisplayLinkCallback>>& callbacks()
 {
-    static HashMap<i32, RefPtr<DisplayLinkCallback>>* map;
-    if (!map)
-        map = new HashMap<i32, RefPtr<DisplayLinkCallback>>;
-    return *map;
+    static HashMap<i32, RefPtr<DisplayLinkCallback>> s_map;
+    return s_map;
 }
 
 static i32 s_next_callback_id = 1;