runtime/cgo: use atomic.Uintptr instead of atomic.AddUintptr.

runtime/cgo: use atomic.Uintptr instead of atomic.AddUintptr.

cgo.NewHandle atomically increments a global uintptr index using
atomic.AddUintptr. Use atomic.Uintptr instead, which is
cleaner and clearer.

Change-Id: I845b3e4cb8c461e787a9b9bb2a9ceaaef1d21d8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/490775
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>