syscall: use unsafe.Slice in anyToSockaddr

The function was added since go1.17, which is the minimum version for
bootstraping now.

Change-Id: I08b55c3639bb9ff042aabfcdcfbdf2993032ba6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/471436


Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>