kernel/thread: Use owner_process when setting the page table in SetupMainThread()

The owning process of a thread is required to exist before the thread,
so we can enforce this API-wise by using a reference. We can also avoid
the reliance on the system instance by using that parameter to access
the page table that needs to be set.

src/core/hle/kernel/thread.h

@@ -281,7 +281,7 @@ private:
  * @return A shared pointer to the main thread
  */
 SharedPtr<Thread> SetupMainThread(KernelCore& kernel, VAddr entry_point, u32 priority,
-                                  SharedPtr<Process> owner_process);
+                                  Process& owner_process);
 
 /**
  * Gets the current thread