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/race.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build race
+// +build plan9,race
 
 package plan9
 

plan9/race0.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !race
+// +build plan9,!race
 
 package plan9
 

plan9/str.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
 
 func itoa(val int) string { // do it here rather than with fmt to avoid dependency

plan9/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 plan9
+
 // Package plan9 contains an interface to the low-level operating system
 // primitives.  OS details vary depending on the underlying system, and
 // by default, godoc will display the OS-specific documentation for the current

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 (