rt: Change the way the kernel exits to avoid pthread leaks

This makes the kernel join every scheduler thread before exiting in order to
ensure that all threads are completely terminated before the process exits. On
my machine, for 32-bit targets, this was causing regular valgrind errors.

src/rt/rust_task_thread.cpp

@@ -320,7 +320,6 @@ rust_task_thread::create_task(rust_task *spawner, const char *name,
 
 void rust_task_thread::run() {
     this->start_main_loop();
-    detach();
     sched->release_task_thread();
 }