runtime: let (*sigctxt).r9 return the correct register value on netbsd

runtime: let (*sigctxt).r9 return the correct register value on netbsd

Fixes #59152

Change-Id: I7f89e2b48db9ff120d7cd094b72ded865874e961
Reviewed-on: https://go-review.googlesource.com/c/go/+/478135
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>