UserspaceEmulator+LibC: Use sys$emuctl() to pass malloc info to UE

Get rid of the awkward secret handshake sequence between malloc and UE
and simply use sys$emuctl() to notify UE of malloc, free and realloc.

Userland/DevTools/UserspaceEmulator/Emulator.h

@@ -82,7 +82,7 @@ private:
     void register_signal_handlers();
     void setup_signal_trampoline();
 
-    int virt$emuctl();
+    int virt$emuctl(FlatPtr, FlatPtr, FlatPtr);
     int virt$fork();
     int virt$execve(FlatPtr);
     int virt$access(FlatPtr, size_t, int);