CL 49590 made it possible for external signal handlers to catch
signals from a crashing Go process. This CL extends that support
to handlers registered after the Go runtime has initialized.

Updates #20392 (and possibly fix it).

Change-Id: I18eccd5e958a505f4d1782a7fc51c16bd3a4ff9c
Reviewed-on: https://go-review.googlesource.com/57291
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>