Kernel: Fix ASSERTION failed in join_thread syscall

set_interrupted_by_death was never called whenever a thread that had
a joiner died, so the joiner remained with the joinee pointer there,
resulting in an assertion fail in JoinBlocker: m_joinee pointed to
a freed task, filled with garbage.

Thread::current->m_joinee may not be valid after the unblock

Properly return the joinee exit value to the joiner thread.

Kernel/Thread.cpp

@@ -300,6 +300,7 @@ void Thread::finalize()
     if (m_joiner) {
         ASSERT(m_joiner->m_joinee == this);
         static_cast<JoinBlocker*>(m_joiner->m_blocker)->set_joinee_exit_value(m_exit_value);
+        static_cast<JoinBlocker*>(m_joiner->m_blocker)->set_interrupted_by_death();
         m_joiner->m_joinee = nullptr;
         // NOTE: We clear the joiner pointer here as well, to be tidy.
         m_joiner = nullptr;