This fixes the problems with signal handling that were inadvertently
introduced in https://go-review.googlesource.com/21006.

Fixes #14899

Change-Id: Ia746914dcb3146a52413d32c57b089af763f0810
Reviewed-on: https://go-review.googlesource.com/21145Reviewed-by: Ian Lance Taylor <iant@golang.org>