cmd/internal/obj/arm64: add NOOP description in Go assembly syntax

This Patch describes NOOP in Go assembly syntax and gives Go assembly
example and corresponding GNU assembly example.

Change-Id: I9db659cc5e3dc6b1f1450f2064255af8872d4b1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/207400


Run-TryBot: eric fang <eric.fang@arm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>