Kernel: Generate coredump backtraces from "threads for coredump" list

This broke with the change that gave each process a list of its own
threads. Since threads are removed slightly earlier from that list
during process teardown, we're not able to use it for generating
coredump backtraces. Fortunately we have the "threads for coredump"
list for just this purpose. :^)

Kernel/Process.cpp

@@ -677,7 +677,7 @@ void Process::die()
     m_tty = nullptr;
 
     for_each_thread([&](auto& thread) {
-        m_threads_for_coredump.append(&thread);
+        m_threads_for_coredump.append(thread);
         return IterationDecision::Continue;
     });