os: use atomic.Uintptr for Process.handle

os: use atomic.Uintptr for Process.handle

Suggested-by: Michael Knyszek <mknyszek@google.com>
Change-Id: I116731b6c3738aae8ff1d3be227f8f51fa3320c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/544795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>