The sigInitIgnored function can be called by initsig before a shared
library is initialized, before the runtime is initialized.

Fixes #27183

Change-Id: I7073767938fc011879d47ea951d63a14d1cce878
Reviewed-on: https://go-review.googlesource.com/131277
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>