[release-branch.go1.8] cmd/link: skip TestDWARF when cgo is disabled

While we're here, fix a Skip/Skipf error I noticed.

Fixes #19796.

(This fixes failures on the release branch introduced by cherry-pick
CL 39605.)

Change-Id: I59b1f5b5ea727fc314acfee8445b3de0b5af1e46
Reviewed-on: https://go-review.googlesource.com/39612
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/dwarf_test.go

@@ -24,6 +24,7 @@ func TestDWARF(t *testing.T) {
 		t.Skip("DWARF is not supported on Windows")
 	}
 
+	testenv.MustHaveCGO(t)
 	testenv.MustHaveGoBuild(t)
 
 	if runtime.GOOS == "plan9" {

src/cmd/link/linkbig_test.go

@@ -21,7 +21,7 @@ import (
 
 func TestLargeText(t *testing.T) {
 	if testing.Short() || (obj.GOARCH != "ppc64le" && obj.GOARCH != "ppc64" && obj.GOARCH != "arm") {
-		t.Skip("Skipping large text section test in short mode or on %s", obj.GOARCH)
+		t.Skipf("Skipping large text section test in short mode or on %s", obj.GOARCH)
 	}
 	testenv.MustHaveGoBuild(t)
 

src/internal/testenv/testenv.go

@@ -138,6 +138,15 @@ func MustHaveExternalNetwork(t *testing.T) {
 	}
 }
 
+var haveCGO bool
+
+// MustHaveCGO calls t.Skip if cgo is not available.
+func MustHaveCGO(t *testing.T) {
+	if !haveCGO {
+		t.Skipf("skipping test: no cgo")
+	}
+}
+
 // HasSymlink reports whether the current system can use os.Symlink.
 func HasSymlink() bool {
 	ok, _ := hasSymlink()

src/internal/testenv/testenv_cgo.go

@@ -0,0 +1,11 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build cgo
+
+package testenv
+
+func init() {
+	haveCGO = true
+}