os: employ sendfile(2) for file-to-file copying on SunOS when needed

os: employ sendfile(2) for file-to-file copying on SunOS when needed

Change-Id: Ia46de6c62707db9ef193fe1a2aabb18585f1dd48
Reviewed-on: https://go-review.googlesource.com/c/go/+/603098
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>