go.sys: fix imports to cgc/p/go.sys, add build tags
Approach but probably not achieve a green build.
LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/126120043
diff --git a/unix/syscall.go b/unix/syscall.go
index 4472ad8..7ba4475 100644
--- a/unix/syscall.go
+++ b/unix/syscall.go
@@ -2,6 +2,8 @@
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
// Package unix contains an interface to the low-level operating system
// primitives. OS details vary depending on the underlying system, and
// by default, godoc will display OS-specific documentation for the current
