kernel: Make handle type declarations constexpr

Some objects declare their handle type as const, while others declare it
as constexpr. This makes the const ones constexpr for consistency, and
prevent unexpected compilation errors if these happen to be attempted to be
used within a constexpr context.

src/core/hle/kernel/readable_event.h

@@ -31,7 +31,7 @@ public:
         return reset_type;
     }
 
-    static const HandleType HANDLE_TYPE = HandleType::ReadableEvent;
+    static constexpr HandleType HANDLE_TYPE = HandleType::ReadableEvent;
     HandleType GetHandleType() const override {
         return HANDLE_TYPE;
     }