Parsing literal IPv6 address with zone identifier is already supported
when not using cgo. This change enables it when using cgo too.

Fixes #12241.

Change-Id: I3ed78c9e750e75eff0dae76ba8608df39503cf85
Reviewed-on: https://go-review.googlesource.com/17215
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>