cmd/asm: add asimd instruction 'rev16' on arm64

cmd/asm: add asimd instruction 'rev16' on arm64

Add support to the asimd instruction rev16 which reverses elements in
16-bit halfwords.

syntax:
	VREV16 <Vn>.<T>, <Vd>.<T>
<T> should be either B8 or B16.

Change-Id: I7a7b8e772589c51ca9eb6dca98bab1aac863c6c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/213738
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>