posix: Consolidate fork implementation

The Linux nptl implementation is used as base for generic fork
implementation to handle the internal locks and mutexes.  The
system specific bits are moved a new internal _Fork symbol.

(This new implementation will be used to provide a async-signal-safe
_Fork now that POSIX has clarified that fork might not be
async-signal-safe [1]).

For Hurd it means that the __nss_database_fork_prepare_parent and
__nss_database_fork_subprocess will be run in a slight different
order.

[1] https://austingroupbugs.net/view.php?id=62

sysdeps/generic/fork.h

@@ -0,0 +1,32 @@
+/* System specific fork hooks.  Generic version.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   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.
+
+   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/>.  */
+
+static inline void
+fork_system_setup (void)
+{
+}
+
+static inline void
+fork_system_setup_after_fork (void)
+{
+}
+
+static inline void
+reclaim_stacks (void)
+{
+}