[kernel] Add thread_join syscall

Thread joining is an important primitive that I seem to have totally
forgotten to implement previously.

src/kernel/syscalls/thread.cpp

@@ -36,6 +36,12 @@ thread_kill(thread *self)
     return j6_status_ok;
 }
 
+j6_status_t
+thread_join(thread *self)
+{
+    return self->join();
+}
+
 j6_status_t
 thread_exit()
 {