cmd/ld: don't emit unreachable dynimport symbols in ELF symtab.

Fix build for Dragonfly BSD.
Fixes issue 7318.
Fixes issue 7367.

LGTM=jsing, iant
R=jsing, iant, mikioh.mikioh
CC=golang-codereviews
https://codereview.appspot.com/64340043

2 files changed, 21 insertions, 0 deletions

diff --git a/misc/cgo/testso/cgoso_c.c b/misc/cgo/testso/cgoso_c.c
index 27155c27f..9b77a76fc 100644
--- a/misc/cgo/testso/cgoso_c.c
+++ b/misc/cgo/testso/cgoso_c.c
@@ -17,6 +17,7 @@ __declspec(dllexport) void sofunc(void);
 #else
 extern void goCallback(void);
 void setCallback(void *f) { (void)f; }
+__thread int tlsvar = 12345;
 #endif
 
 void sofunc(void)
diff --git a/misc/cgo/testso/cgoso_unix.go b/misc/cgo/testso/cgoso_unix.go
new file mode 100644
index 000000000..e86f99264
--- /dev/null
+++ b/misc/cgo/testso/cgoso_unix.go
@@ -0,0 +1,20 @@
+// 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.
+
+// +build darwin dragonfly freebsd linux netbsd
+
+package cgosotest
+
+/*
+extern int __thread tlsvar;
+int *getTLS() { return &tlsvar; }
+*/
+import "C"
+
+func init() {
+	if v := *C.getTLS(); v != 12345 {
+		println("got", v)
+		panic("BAD TLS value")
+	}
+}