syscall: convert Windows AddrinfoW.Addr from uintptr to syscall.Pointer

syscall: convert Windows AddrinfoW.Addr from uintptr to syscall.Pointer

API compatibility violation, but it wasn't safe before and people
should be using golang.org/x/sys/windows instead.

Fixes #24820

Change-Id: I3b43493f56b3116924b9d848a294899279f314fd
Reviewed-on: https://go-review.googlesource.com/123455
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>