Implement stack growth for system calls and add stack pointer tracking to thread

src/threads/thread.h

@@ -143,6 +143,10 @@ struct thread
     struct hash open_files;             /* Hash Table of FD -> Struct File. */
 #endif
 
+#ifdef VM
+    void *curr_esp;
+#endif
+
     /* Owned by thread.c. */
     unsigned magic;                     /* Detects stack overflow. */
   };