cmd/compile/internal/ssa: intrinsify atomic.Xchg8 on amd64

cmd/compile/internal/ssa: intrinsify atomic.Xchg8 on amd64

For #68578

Change-Id: Ia9580579bfc4709945bfcf6ec3803d5d11812187
Reviewed-on: https://go-review.googlesource.com/c/go/+/606901
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>