runtime/cgo: only build _cgo_callers if x_cgo_callers is defined

Fixes a problem when using the external linker on Solaris.  The Solaris
external linker still doesn't work due to issue #14957.

The problem is, for example, with `go test cmd/objdump`:

        objdump_test.go:71: go build fmthello.go: exit status 2
                # command-line-arguments
                /var/gcc/iant/go/pkg/tool/solaris_amd64/link: running gcc failed: exit status 1
                Undefined                       first referenced
                 symbol                             in file
                x_cgo_callers                       /tmp/go-link-355600608/go.o
                ld: fatal: symbol referencing errors
                collect2: error: ld returned 1 exit status

Change-Id: I54917cfd5c288ee77ea25c439489bd2c9124fe73
Reviewed-on: https://go-review.googlesource.com/21392
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/cgo/callbacks.go

@@ -92,13 +92,5 @@ var _cgo_sys_thread_create = &x_cgo_sys_thread_create
 var x_cgo_notify_runtime_init_done byte
 var _cgo_notify_runtime_init_done = &x_cgo_notify_runtime_init_done
 
-// Calls the traceback function passed to SetCgoTraceback.
-
-//go:cgo_import_static x_cgo_callers
-//go:linkname x_cgo_callers x_cgo_callers
-//go:linkname _cgo_callers _cgo_callers
-var x_cgo_callers byte
-var _cgo_callers = &x_cgo_callers
-
 //go:cgo_export_static _cgo_topofstack
 //go:cgo_export_dynamic _cgo_topofstack

src/runtime/cgo/callbacks_traceback.go

+// Copyright 2016 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 linux
+
+package cgo
+
+import _ "unsafe" // for go:linkname
+
+// Calls the traceback function passed to SetCgoTraceback.
+
+//go:cgo_import_static x_cgo_callers
+//go:linkname x_cgo_callers x_cgo_callers
+//go:linkname _cgo_callers _cgo_callers
+var x_cgo_callers byte
+var _cgo_callers = &x_cgo_callers