[kernel] Add object_wait_many syscall

Add the object_wait_many syscall to allow programs to wait for signals
on multiple objects at once. Also removed the object argument to
thread::wait_on_signals, which does nothing with it. That information is
saved in the thread being in the object's blocked threads list.

src/kernel/syscalls/thread.cpp

@@ -40,7 +40,7 @@ j6_status_t
 thread_pause()
 {
 	thread &th = thread::current();
-	th.wait_on_signals(&th, -1ull);
+	th.wait_on_signals(-1ull);
 	return j6_status_ok;
 }