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

plan9/syscall_test.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 plan9
+
 package plan9_test
 
 import (