AK: Add global FlatPtr typedef. It's u32 or u64, based on sizeof(void*)

Use this instead of uintptr_t throughout the codebase. This makes it
possible to pass a FlatPtr to something that has u32 and u64 overloads.

DevTools/ProfileViewer/Profile.h

@@ -120,7 +120,7 @@ public:
     struct Event {
         u64 timestamp { 0 };
         String type;
-        uintptr_t ptr { 0 };
+        FlatPtr ptr { 0 };
         size_t size { 0 };
         bool in_kernel { false };
         Vector<Frame> frames;