syscall: use unsafe.Slice in (*RawSockaddrAny).Sockaddr on windows

syscall: use unsafe.Slice in (*RawSockaddrAny).Sockaddr on windows

Follow CL 471436.

Change-Id: I415b126f58dbd381b8d45457e7bd79f025a2a03c
Reviewed-on: https://go-review.googlesource.com/c/go/+/472035
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>