Updated makefile and removed exit_thread() from syscall.c

Makefile.build

@@ -63,6 +63,7 @@ userprog_SRC += userprog/syscall.c	# System call handler.
 userprog_SRC += userprog/gdt.c		# GDT initialization.
 userprog_SRC += userprog/tss.c		# TSS management.
 userprog_SRC += userprog/syscall_exec.c
+userprog_SRC += userprog/syscall_exit.c
 userprog_SRC += userprog/syscall_open.c
 userprog_SRC += userprog/syscall_halt.c
 userprog_SRC += userprog/syscall_wait.c

userprog/syscall.c

@@ -60,7 +60,6 @@ syscall_handler (struct intr_frame *f UNUSED)
     break;
   case SYSCALL_EXIT:
     syscall_exit(f);
-    thread_exit();
     break;
   case SYSCALL_EXEC:
     syscall_exec(f);