util/racebuild: add package to export a race-is-enabled const

util/racebuild/racebuild.go

@@ -0,0 +1,7 @@
+// Copyright (c) 2020 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.
+
+// Package racebuild exports a constant about whether the current binary
+// was built with the race detector.
+package racebuild