internal/runtime/atomic: add Xchg8 for arm
For #69735
Change-Id: I18c0ca15d94a9b1751c1e55459283e01dc114150
GitHub-Last-Rev:
dd9a39a5551e5a3415ab765cf271fecdbbe89b4c
GitHub-Pull-Request: golang/go#69924
Cq-Include-Trybots: luci.golang.try:gotip-linux-arm
Reviewed-on: https://go-review.googlesource.com/c/go/+/620855
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>