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/Devices/SB16.h

@@ -42,6 +42,7 @@ public:
     SB16();
     virtual ~SB16() override;
 
+    static void create();
     static SB16& the();
 
     // ^CharacterDevice