Consolidate pthread_atfork

The pthread_atfork is similar between Linux and Hurd, only the compat
version bits differs.  The generic version is place at sysdeps/pthread
with a common name.

It also fixes an issue with Hurd license, where the static-only object
did not use LGPL + exception.

Checked on x86_64-linux-gnu, i686-linux-gnu, and with a build for
i686-gnu.

sysdeps/pthread/pthread_atfork.c

@@ -0,0 +1,58 @@
+/* Copyright (C) 2002-2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   In addition to the permissions in the GNU Lesser General Public
+   License, the Free Software Foundation gives you unlimited
+   permission to link the compiled version of this file with other
+   programs, and to distribute those programs without any restriction
+   coming from the use of this file. (The GNU Lesser General Public
+   License restrictions do apply in other respects; for example, they
+   cover modification of the file, and distribution when not linked
+   into another program.)
+
+   Note that people who make modified versions of this file are not
+   obligated to grant this special exception for their modified
+   versions; it is their choice whether to do so. The GNU Lesser
+   General Public License gives permission to release a modified
+   version without this exception; this exception also makes it
+   possible to release a modified version which carries forward this
+   exception.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include "pthreadP.h"
+#include <register-atfork.h>
+#include <dso_handle.h>
+#include <register-atfork.h>
+
+
+/* Hide the symbol so that no definition but the one locally in the
+   executable or DSO is used.  */
+int
+#ifndef __pthread_atfork
+/* Don't mark the compatibility function as hidden.  */
+attribute_hidden
+#endif
+__pthread_atfork (void (*prepare) (void), void (*parent) (void),
+		  void (*child) (void))
+{
+  return __register_atfork (prepare, parent, child, __dso_handle);
+}
+#ifndef __pthread_atfork
+extern int pthread_atfork (void (*prepare) (void), void (*parent) (void),
+			   void (*child) (void)) attribute_hidden;
+weak_alias (__pthread_atfork, pthread_atfork)
+#endif