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/BXVGADevice.h

@@ -36,6 +36,7 @@ namespace Kernel {
 class BXVGADevice final : public BlockDevice {
     AK_MAKE_ETERNAL
 public:
+    static void initialize();
     static BXVGADevice& the();
 
     BXVGADevice();