go.toolchain.rev: add Go toolchain rev, tool to print it out

Updates tailscale/corp#3385

Change-Id: Ia0e285a0ae836744539c97ff6eff207588159688
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/printdep/printdep.go

@@ -0,0 +1,26 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The printdep command is a build system tool for printing out information
+// about dependencies.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+
+	ts "tailscale.com"
+)
+
+var (
+	goToolchain = flag.Bool("go", false, "print the supported Go toolchain git hash (a github.com/tailscale/go commit)")
+)
+
+func main() {
+	flag.Parse()
+	if *goToolchain {
+		fmt.Println(strings.TrimSpace(ts.GoToolchainRev))
+	}
+}

go.toolchain.rev

@@ -0,0 +1 @@
+118f76c18c3f755ba0e6211e170a6422871f2791

version-embed.go

@@ -9,3 +9,9 @@
 
 //go:embed VERSION.txt
 var Version string
+
+// GoToolchainRev is the git hash from github.com/tailscale/go that this release
+// should be built using. It may end in a newline.
+//
+//go:embed go.toolchain.rev
+var GoToolchainRev string