internal/runtime/atomic: add Xchg8 for 386
For #69735
Change-Id: I5b9f57315d693d613dc88dc02c10bee39aeeef76
GitHub-Last-Rev:
690337e5b81a48bdcb808526d0c5f4837e8912b7
GitHub-Pull-Request: golang/go#69923
Reviewed-on: https://go-review.googlesource.com/c/go/+/620756
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>