Before this CL, whenever the Go runtime wanted to kill its own
process with a signal dieFromSignal would reset the signal handler
to _SIG_DFL.

Unfortunately, if any signal handler were installed before the Go
runtime initialized, it wouldn't be invoked either.

Instead, use whatever signal handler was installed before
initialization.

The motivating use case is Crashlytics on Android. Before this CL,
Crashlytics would not consider a crash from a panic() since the
corresponding SIGABRT never reached its signal handler.

Updates #11382
Updates #20392 (perhaps even fixes it)
Fixes #19389

Change-Id: I0c8633329433b45cbb3b16571bea227e38e8be2e
Reviewed-on: https://go-review.googlesource.com/49590
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>