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.go

+// 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.
+
+// Test that -static works when not using cgo.  This test is in
+// misc/cgo to take advantage of the testing framework support for
+// when -static is expected to work.
+
+package nocgo
+
+func NoCgo() int {
+	c := make(chan int)
+
+	// The test is run with external linking, which means that
+	// goroutines will be created via the runtime/cgo package.
+	// Make sure that works.
+	go func() {
+		c <- 42
+	}()
+
+	return <-c
+}

misc/cgo/nocgo/nocgo_test.go

+// 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)
+	}
+}

src/cmd/ld/lib.c

@@ -205,6 +205,8 @@ loadlib(void)
 		// whether to initialize the TLS.  So give it one.  This could
 		// be handled differently but it's an unusual case.
 		loadinternal("runtime/cgo");
+		if(i < ctxt->libraryp)
+			objfile(ctxt->library[i].file, ctxt->library[i].pkg);
 
 		// Pretend that we really imported the package.
 		s = linklookup(ctxt, "go.importpath.runtime/cgo.", 0);

src/run.bash

@@ -145,6 +145,9 @@ dragonfly-386 | dragonfly-amd64 | freebsd-386 | freebsd-amd64 | freebsd-arm | li
 			echo "No support for static linking found (lacks libc.a?), skip cgo static linking test."
 		else
 			go test -ldflags '-linkmode=external -extldflags "-static -pthread"' ../testtls || exit 1
+			go test ../nocgo || exit 1
+			go test -ldflags '-linkmode=external' ../nocgo || exit 1
+			go test -ldflags '-linkmode=external -extldflags "-static -pthread"' ../nocgo || exit 1
 		fi
 		;;
 	esac