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/creds_test.go

@@ -11,7 +11,8 @@ import (
 	"net"
 	"os"
 	"testing"
-	"unix"
+
+	"code.google.com/p/go.sys/unix"
 )
 
 // TestSCMCredentials tests the sending and receiving of credentials