net/netip: add IPv6LinkLocalAllRouters and IPv6Loopback

net/netip: add IPv6LinkLocalAllRouters and IPv6Loopback

Fixes #51766
Fixes #51777

Change-Id: I0510175c20c06442d78b2581cfe218e66be1c35b
Reviewed-on: https://go-review.googlesource.com/c/go/+/412475
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>