Apparently when GOMAXPROCS == 1 a simple sched_yield in a tight loop is
not necessarily sufficient to permit a signal handler to run. Instead,
sleep for 1/1000 of a second.

Fixes #16649.

Change-Id: I83910144228556e742b7a92a441732ef61aa49d9
Reviewed-on: https://go-review.googlesource.com/33239
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>