cmd/cover: fix build

Fix the various builds which don't have a real filesystem or don't support forking.

Change-Id: I3075c662fe6191ecbe70ba359b73d9a88bb06f35
Reviewed-on: https://go-review.googlesource.com/9528Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/cover/cover_test.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"testing"
 )
 
@@ -40,6 +41,15 @@ var debug = false // Keeps the rewritten files around if set.
 //	go run ./testdata/main.go ./testdata/test.go
 //
 func TestCover(t *testing.T) {
+	switch runtime.GOOS {
+	case "nacl":
+		t.Skipf("skipping; %v/%v no support for forking", runtime.GOOS, runtime.GOARCH)
+	case "darwin", "android":
+		switch runtime.GOARCH {
+		case "arm", "arm64":
+			t.Skipf("skipping; %v/%v no support for forking", runtime.GOOS, runtime.GOARCH)
+		}
+	}
 	// Read in the test file (testTest) and write it, with LINEs specified, to coverInput.
 	file, err := ioutil.ReadFile(testTest)
 	if err != nil {