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/FileSystem/VirtualFileSystem.h

@@ -78,6 +78,7 @@ public:
         int m_flags;
     };
 
+    static void initialize();
     static VFS& the();
 
     VFS();