cmd/go: always use --whole-archive for gccgo packages

This is, in effect, what the gc toolchain does.  It fixes cases where Go
code refers to a C global variable; without this, if the global variable
was the only thing visible in the C code, the generated cgo file might
not get pulled in from the archive, leaving the Go variable
uninitialized.

This was reported against gccgo as https://gcc.gnu.org/PR68255 .

Change-Id: I3e769dd174f64050ebbff268fbbf5e6fab1e2a1b
Reviewed-on: https://go-review.googlesource.com/16775
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

misc/cgo/test/cgo_test.go

@@ -66,5 +66,6 @@ func Test9557(t *testing.T)                  { test9557(t) }
 func Test10303(t *testing.T)                 { test10303(t, 10) }
 func Test11925(t *testing.T)                 { test11925(t) }
 func Test12030(t *testing.T)                 { test12030(t) }
+func TestGCC68255(t *testing.T)              { testGCC68255(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/gcc68255.go

+// Copyright 2015 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
+
+import (
+	"testing"
+
+	"./gcc68255"
+)
+
+func testGCC68255(t *testing.T) {
+	if !gcc68255.F() {
+		t.Error("C global variable was not initialized")
+	}
+}

misc/cgo/test/gcc68255/a.go

+// Copyright 2015 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 it's OK to have C code that does nothing other than
+// initialize a global variable.  This used to fail with gccgo.
+
+package gcc68255
+
+/*
+#include "c.h"
+*/
+import "C"
+
+func F() bool {
+	return C.v != nil
+}

misc/cgo/test/gcc68255/c.c

+// Copyright 2015 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 void f(void) {
+}
+
+void (*v)(void) = f;

misc/cgo/test/gcc68255/c.h

+// Copyright 2015 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.
+
+extern void (*v)(void);

src/cmd/go/build.go

@@ -2599,17 +2599,9 @@ func (tools gccgoToolchain) ld(b *builder, root *action, out string, allactions
 		}
 	}
 
-	switch ldBuildmode {
-	case "c-archive", "c-shared":
 	ldflags = append(ldflags, "-Wl,--whole-archive")
-	}
-
 	ldflags = append(ldflags, afiles...)
-
-	switch ldBuildmode {
-	case "c-archive", "c-shared":
 	ldflags = append(ldflags, "-Wl,--no-whole-archive")
-	}
 
 	ldflags = append(ldflags, cgoldflags...)
 	ldflags = append(ldflags, envList("CGO_LDFLAGS", "")...)