cmd/objdump: don't run TestDisasmExtld if cgo is not enabled

The test uses external linking mode, which is probably not available
if cgo does not work.

Fixes #11969.

Change-Id: Id1c2828cd2540391e16b422bf51674ba6ff084b0
Reviewed-on: https://go-review.googlesource.com/13005Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/objdump/objdump_test.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"go/build"
 	"internal/testenv"
 	"io/ioutil"
 	"os"
@@ -123,5 +124,8 @@ func TestDisasmExtld(t *testing.T) {
 	if runtime.GOOS == "openbsd" && runtime.GOARCH == "arm" {
 		t.Skip("skipping on openbsd/arm, no support for external linking, issue 10619")
 	}
+	if !build.Default.CgoEnabled {
+		t.Skip("skipping because cgo is not enabled")
+	}
 	testDisasm(t, "-ldflags=-linkmode=external")
 }