cmd/viewcore: skip build on plan9

./cmd/viewcore depends on github.com/chzyer/readline, which
doesn't build on Plan 9.

Updates golang/go#32839

Change-Id: I5e0638a65914dcad7e7c9383df77e24a6b616a89
Reviewed-on: https://go-review.googlesource.com/c/debug/+/229877
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/viewcore/html.go

diff --git a/cmd/viewcore/html.go b/cmd/viewcore/html.go
index 99b132e..a39ebef 100644
--- a/cmd/viewcore/html.go
+++ b/cmd/viewcore/html.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 !aix
+// +build !aix,!plan9
 // (https://golang.org/issue/32839)
 
 package main

cmd/viewcore/main.go

diff --git a/cmd/viewcore/main.go b/cmd/viewcore/main.go
index 8d5c953..7cc9e3c 100644
--- a/cmd/viewcore/main.go
+++ b/cmd/viewcore/main.go
@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Skip aix for now: github.com/chzyer/readline doesn't support it.
+// Skip aix and plan9 for now: github.com/chzyer/readline doesn't support them.
 // (https://golang.org/issue/32839)
 //
-// +build !aix
+// +build !aix,!plan9
 
 // The viewcore tool is a command-line tool for exploring the state of a Go process
 // that has dumped core.