kern: devirtualize KAutoObject::DynamicCast<>()

This is an optimization that saves the most common type of virtual call in the kernel (DynamicCast)
by storing class token as a member, rather than getting it via virtual call every time.

This does not currently cost any memory space on 64-bit targets, due to pre-existing padding space.

This optimization can be turned off via a compile-time flag for accuracy.

libraries/libmesosphere/include/mesosphere/kern_build_config.hpp

@@ -32,3 +32,10 @@
 //#define MESOSPHERE_BUILD_FOR_TRACING
 #define MESOSPHERE_ENABLE_PANIC_REGISTER_DUMP
 #define MESOSPHERE_ENABLE_HARDWARE_SINGLE_STEP
+
+/* NOTE: This enables fast class token storage using a class member. */
+/* This saves a virtual call when doing KAutoObject->DynCast<>(), */
+/* at the cost of storing class tokens inside the class object. */
+/* However, as of (10/16/2021) KAutoObject has an unused class member */
+/* of the right side, and so this does not actually cost any space. */
+#define MESOSPHERE_ENABLE_DEVIRTUALIZED_DYNAMIC_CAST