Other systems use pthread_sigmask.  It was a mistake to use sigprocmask
here.

Change-Id: Ie045aa3f09cf035fcf807b7543b96fa5b847958a
Reviewed-on: https://go-review.googlesource.com/16720Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>