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

unix/syscall_test.go

@@ -2,11 +2,14 @@
 // 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_test
 
 import (
 	"testing"
-	"unix"
+
+	"code.google.com/p/go.sys/unix"
 )
 
 func testSetGetenv(t *testing.T, key, value string) {