internal/runtime/atomic: add Xchg8 to all GOOS=linux platforms

[rhysh]

Pending acceptance of CL 601597 (for #68578), we'll want an atomic.Xchg8 implementation for all GOARCH values that support it — at least those that overlap with GOOS=linux. This issue tracks that work, including expanding the test from CL 606900 to cover those platforms and eventually to move into atomic_test.go.

CC @golang/runtime
CC @mauri870 (who showed interest in doing some of the work)

[gopherbot]

Change https://go.dev/cl/617496 mentions this issue: cmd/compile: add internal/runtime/atomic.Xchg8 intrinsic for PPC64

[mauri870]

Thanks for the ping, Rhys! I have some code ready for ARM64 and will open a CL soon. I'll try to look into the remaining architectures later this week.

[gopherbot]

Change https://go.dev/cl/617595 mentions this issue: cmd/compile, runtime/internal/atomic: add Xchg8 for arm64

[gopherbot]

Change https://go.dev/cl/620756 mentions this issue: internal/runtime/atomic: add Xchg8 for 386