internal/runtime/atomic: add Xchg8 for amd64

internal/runtime/atomic: add Xchg8 for amd64

For #68578

Change-Id: Idecfdbb793f46560dd69287af9170c07cf4ee973
Reviewed-on: https://go-review.googlesource.com/c/go/+/606900
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>