Add sys_hello and testbin/hellotest

`sys_hello` is a new syscall that just prints some text in the terminal.

`testbin/hellotest` is a userland binary that just calls `sys_hello`

kern/arch/mips/syscall/syscall.c

@@ -104,6 +104,9 @@ void syscall(struct trapframe *tf) {
   case SYS___time:
     err = sys___time((userptr_t)tf->tf_a0, (userptr_t)tf->tf_a1);
     break;
+  case SYS_hello:
+    err = sys_hello();
+    break;
 
   case SYS__exit:
     err = 0;