[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/objects/thread.cpp

@@ -48,7 +48,7 @@ thread & thread::current() { return *current_cpu().thread; }
 inline void schedule_if_current(thread *t) { if (t == current_cpu().thread) scheduler::get().schedule(); }
 
 void
-thread::wait_on_signals(kobject *obj, j6_signal_t signals)
+thread::wait_on_signals(j6_signal_t signals)
 {
 	m_wait_type = wait_type::signal;
 	m_wait_data = signals;