internal/runtime/atomic: add Xchg8 for riscv64

internal/runtime/atomic: add Xchg8 for riscv64

For #69735

Change-Id: I34ca2b027494525ab64f94beee89ca373a5031ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/631615
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>