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/mmap_unix_test.go

@@ -8,7 +8,8 @@ package unix_test
 
 import (
 	"testing"
-	"unix"
+
+	"code.google.com/p/go.sys/unix"
 )
 
 func TestMmap(t *testing.T) {