runtime/cgo: export _cgo_reginit on ppc64x

This is needed to make external linking work.

Change-Id: I4cf7edb4ea318849cab92a697952f8745eed40c4
Reviewed-on: https://go-review.googlesource.com/14237Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo_ppc64x.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 runtime
+
+// +build ppc64 ppc64le
+
+// crosscall_ppc64 calls into the runtime to set up the registers the
+// Go runtime expects and so the symbol it calls needs to be exported
+// for external linking to work.
+//go:cgo_export_static _cgo_reginit