[release-branch.go1.12] cmd/internal/obj/arm64: fix the bug assembling TSTW

Current assembler reports error when it assembles
"TSTW $1689262177517664, R3", but go1.11 was building
fine.

Fixes #30334

Change-Id: I9c16d36717cd05df2134e8eb5b17edc385aff0a9
Reviewed-on: https://go-review.googlesource.com/c/163259


Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ben Shi <powerman1st@163.com>
(cherry picked from commit 2ef8abb4)
Reviewed-on: https://go-review.googlesource.com/c/163419


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>