cmd/cgo: fix a problem with 'go build -compiler gccgo'

Port of https://golang.org/cl/154360045 to Git.
Original author is Xia Bin <[email protected]> (already a contributor).

Fixes #8945.

Change-Id: I28bcaf3348794202ca59fbc3466bd7b9670030e4
Reviewed-on: https://go-review.googlesource.com/11658
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc

misc/cgo/test/issue8945.go

@@ -0,0 +1,16 @@
+// 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 gccgo
+
+package cgotest
+
+//typedef void (*PFunc)();
+//PFunc success_cb;
+import "C"
+
+//export Test
+func Test() {
+	_ = C.success_cb
+}

src/cmd/cgo/out.go

@@ -118,11 +118,11 @@ func (p *Package) writeDefs() {
 		}
 
 		if !cVars[n.C] {
-			fmt.Fprintf(fm, "extern char %s[];\n", n.C)
-			fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
 			if *gccgo {
 				fmt.Fprintf(fc, "extern byte *%s;\n", n.C)
 			} else {
+				fmt.Fprintf(fm, "extern char %s[];\n", n.C)
+				fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
 				fmt.Fprintf(fgo2, "//go:linkname __cgo_%s %s\n", n.C, n.C)
 				fmt.Fprintf(fgo2, "//go:cgo_import_static %s\n", n.C)
 				fmt.Fprintf(fgo2, "var __cgo_%s byte\n", n.C)