This addresses the case of a -timeout panic, but not the more
general case of a signal arriving. See CL 48370 and CL 44352
for recent difficulties in that area.

"-timeout" here means flag usage to distinguish from the
default timeout termination which uses signals.

Fixes #19394

Change-Id: I5452d5422c0c080e940cbcc8c6606049975268c6
Reviewed-on: https://go-review.googlesource.com/48491Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>