cmd/asm: add arm64 v8.1 atomic instructions

cmd/asm: add arm64 v8.1 atomic instructions

This change adds several arm64 v8.1 atomic instructions and test cases.
They are LDADDAx, LDADDLx, LDANDAx, LDANDALx, LDANDLx, LDEORAx, LDEORALx,
LDEORLx, LDORAx, LDORALx, LDORLx, SWPAx and SWPLx. Their form is consistent
with the form of the existing atomic instructions.

For instructions STXRx, STLXRx, STXPx and STLXPx, the second destination
register can't be RSP. This CL also adds a check for this.

LDADDx Rs, (Rb), Rt: *Rb -> Rt, Rs + *Rb -> *Rb
LDANDx Rs, (Rb), Rt: *Rb -> Rt, Rs AND NOT(*Rb) -> *Rb
LDEORx Rs, (Rb), Rt: *Rb -> Rt, Rs EOR *Rb -> *Rb
LDORx  Rs, (Rb), Rt: *Rb -> Rt, Rs OR *Rb -> *Rb

Change-Id: I9f9b0245958cb57ab7d88c66fb9159b23b9017fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/157001
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>