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

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

LGTM=jsing, iant
R=jsing, iant, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/64340043

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)

misc/cgo/testso/cgoso_unix.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.
+
+// +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")
+	}
+}

src/cmd/ld/data.c

@@ -312,6 +312,8 @@ dynrelocsym(LSym *s)
 			return;
 		for(r=s->r; r<s->r+s->nr; r++) {
 			targ = r->sym;
+			if(!targ->reachable)
+				diag("internal inconsistency: dynamic symbol %s is not reachable.", targ->name);
 			if(r->sym->plt == -2 && r->sym->got != -2) { // make dynimport JMP table for PE object files.
 				targ->plt = rel->size;
 				r->sym = rel;
@@ -340,9 +342,12 @@ dynrelocsym(LSym *s)
 	}
 
 	for(r=s->r; r<s->r+s->nr; r++) {
-		if(r->sym != S && r->sym->type == SDYNIMPORT || r->type >= 256)
+		if(r->sym != S && r->sym->type == SDYNIMPORT || r->type >= 256) {
+			if(!r->sym->reachable)
+				diag("internal inconsistency: dynamic symbol %s is not reachable.", r->sym->name);
 			adddynrel(s, r);
 		}
+	}
 }
 
 void

src/cmd/ld/symtab.c

@@ -197,7 +197,7 @@ asmelfsym(void)
 	genasmsym(putelfsym);
 	
 	for(s=ctxt->allsym; s!=S; s=s->allsym) {
-		if(s->type != SHOSTOBJ && s->type != SDYNIMPORT)
+		if(s->type != SHOSTOBJ && !(s->type == SDYNIMPORT && s->reachable))
 			continue;
 		if(s->type == SDYNIMPORT)
 			name = s->extname;