Change the two calls to signalstack(nil) to inline the code
instead (it's two lines).

Change-Id: Ie92a05494f924f279e40ac159f1b677fda18f281
Reviewed-on: https://go-review.googlesource.com/29854
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>