Implement proc_spawn_thread syscall, fix proc_resume and proc_suspend

kernel/proc/mutex.h

@@ -8,7 +8,8 @@
 struct proc;
 
 struct proc_mutex {
-  atomic_flag flag;
+  spin_lock_t lock;
+  bool locked;
   struct proc_suspension_q suspension_q;
   struct proc* owner;
 };