[release-branch.go1.11] runtime: mark sigInitIgnored nosplit

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>
(cherry picked from commit d20ecd6e5dab55376ea4f169eed63608f9bb3b2b)
Reviewed-on: https://go-review.googlesource.com/131278


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>