cmd/internal/obj/arm64: fix an encoding error of CMPW instruction

For arm64 CMP, ADD and other similar extended register instructions,
if there is no extension, the default extion is LSL<<0, but the default
encoding value (the value of 'option' field) of 32-bit instruction and
64-bit instruction is different, 32-bit is 2 and 64-bit is 3. But the
current assembler incorrectly encodes the value of 32-bit instruction
to 3. This CL fixes this error.

Change-Id: I0e09af2c9c5047a4ed2db7d1183290283db9c31c
Reviewed-on: https://go-review.googlesource.com/c/go/+/329749


Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: eric fang <eric.fang@arm.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Trust: Dmitri Shuralyov <dmitshur@golang.org>