Kernel: Switch singletons to use new Singleton class

MemoryManager cannot use the Singleton class because
MemoryManager::initialize is called before the global constructors
are run. That caused the Singleton to be re-initialized, causing
it to create another MemoryManager instance.

Fixes #3226

Kernel/Console.h

@@ -34,6 +34,7 @@ class Console final : public Kernel::CharacterDevice {
     AK_MAKE_ETERNAL
 public:
     static Console& the();
+    static void initialize();
     static bool is_initialized();
 
     Console();