runtime: add race annotations in IncNonDefault

runtime: add race annotations in IncNonDefault

Also use CompareAndSwap to make the code actually less racy.

Added a test which will be meaningful when run under the race
detector (tested it -race with broken fix in runtime, it failed).

Fixes #64649

Change-Id: I5972e08901d1adc8ba74858edad7eba91be1b0ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/549796
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>