cmd/ld: really import runtime/cgo for external link

Fixes #8032.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/95580043

misc/cgo/nocgo/nocgo_test.go

diff --git a/misc/cgo/nocgo/nocgo_test.go b/misc/cgo/nocgo/nocgo_test.go
new file mode 100644
index 0000000..45d247c
--- /dev/null
+++ b/misc/cgo/nocgo/nocgo_test.go
@@ -0,0 +1,14 @@
+// Copyright 2014 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.
+
+package nocgo
+
+import "testing"
+
+func TestNop(t *testing.T) {
+	i := NoCgo()
+	if i != 42 {
+		t.Errorf("got %d, want %d", i, 42)
+	}
+}