Kernel: Make sure threads which don't do any syscalls are terminated

Steps to reproduce:

$ cat loop.c
int main() { for (;;); }
$ gcc -o loop loop.c
$ ./loop

Terminating this process wasn't previously possible because we only
checked whether the thread should be terminated on syscall exit.

Kernel/Scheduler.cpp

@@ -192,6 +192,13 @@ bool Scheduler::pick_next()
 
     ScopedSpinLock lock(g_scheduler_lock);
 
+    auto current_thread = Thread::current();
+    if (current_thread->should_die() && current_thread->may_die_immediately()) {
+        // Ordinarily the thread would die on syscall exit, however if the thread
+        // doesn't perform any syscalls we still need to mark it for termination here.
+        current_thread->set_state(Thread::Dying);
+    }
+
     if constexpr (SCHEDULER_RUNNABLE_DEBUG) {
         dump_thread_list();
     }