CPU/Recompiler: Prevent using fastmem when cache is isolated

No point even trying since it's just going to fault.

src/core/cpu_core.h

@@ -94,6 +94,7 @@ void Shutdown();
 void Reset();
 bool DoState(StateWrapper& sw);
 void ClearICache();
+void UpdateFastmemBase();
 
 /// Executes interpreter loop.
 void Execute();