internal/runtime/atomic: add Xchg8 for s390x and wasm
This makes the single-byte atomic.Xchg8 operation available on all
GOARCHes, including those without direct / single-instruction support.
Fixes #69735
Change-Id: Icb6aff8f907257db81ea440dc4d29f96b3cff6c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/657936 Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com>
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