cmd/link: fix imports problem in elf_test.go

Fix an imports problem in this test (doesn't compile).

Updates #35779

Change-Id: Icaeec0384bf2e75696e43d9410df7219f0245940
Reviewed-on: https://go-review.googlesource.com/c/go/+/210578
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/elf_test.go

@@ -15,6 +15,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"sync"
 	"testing"