cmd/ld: handle duplicate static symbols in COFF and Mach-O files.

Fixes #5740.

R=iant, rsc, luisbebop
CC=gobot, golang-dev
https://golang.org/cl/10345046

misc/cgo/test/issue5740.go

+// Copyright 2013 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 cgotest
+
+// int test5740a(void), test5740b(void);
+import "C"
+import "testing"
+
+func test5740(t *testing.T) {
+	if v := C.test5740a() + C.test5740b(); v != 5 {
+		t.Errorf("expected 5, got %v", v)
+	}
+}

misc/cgo/test/issue5740a.c

+// Copyright 2013 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.
+
+static int volatile val = 2;
+
+int test5740a() {
+	return val;
+}

misc/cgo/test/issue5740b.c

+// Copyright 2013 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.
+
+static int volatile val = 3;
+
+int test5740b() {
+	return val;
+}

src/cmd/ld/ldmacho.c

@@ -611,6 +611,8 @@ ldmacho(Biobuf *f, char *pkg, int64 len, char *pn)
 		if(!(sym->type&N_EXT))
 			v = version;
 		s = lookup(name, v);
+		if(!(sym->type&N_EXT))
+			s->dupok = 1;
 		sym->sym = s;
 		if(sym->sectnum == 0)	// undefined
 			continue;

src/cmd/ld/ldpe.c

@@ -469,6 +469,7 @@ readsym(PeObj *obj, int i, PeSym **y)
 		case IMAGE_SYM_CLASS_NULL:
 		case IMAGE_SYM_CLASS_STATIC:
 			s = lookup(name, version);
+			s->dupok = 1;
 			break;
 		default:
 			werrstr("%s: invalid symbol binding %d", sym->name, sym->sclass);