To date, the behavior has depended on whether we're using cgo and
in turn what the host resolver does. Most host resolvers will "resolve"
IP addresses, but the non-cgo pure Go path has not.
This CL makes resolution of IP addresses always work, even if we're not using cgo
and even if the host resolver does not "resolve" IP addresses.

Fixes #11335.

Change-Id: I19e82be968154d94904bb2f72e9c17893019a909
Reviewed-on: https://go-review.googlesource.com/11420Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>