cmd/internal/obj/arm64: add more atomic instructions

More atomic instructions were introduced in ARMv8.1. And this CL
adds support for them and corresponding test cases.

LDADD Rs, (Rb), Rt: (Rb) -> Rt, Rs+(Rb) -> (Rb)
LDAND Rs, (Rb), Rt: (Rb) -> Rt, Rs&(Rb) -> (Rb)
LDEOR Rs, (Rb), Rt: (Rb) -> Rt, Rs^(Rb) -> (Rb)
LDOR  Rs, (Rb), Rt: (Rb) -> Rt, Rs|(Rb) -> (Rb)

Change-Id: Ifb9df86583c4dc54fb96274852c3b93a197045e4
Reviewed-on: https://go-review.googlesource.com/110535


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