malloc: Remove __malloc_initialize_hook from the API [BZ #19564]

__malloc_initialize_hook is interposed by application code, so
the usual approach to define a compatibility symbol does not work.
This commit adds a new mechanism based on #pragma GCC poison in
<stdc-predef.h>.

malloc/malloc-hooks.h

@@ -0,0 +1,27 @@
+/* Internal declarations of malloc hooks no longer in the public API.
+   Copyright (C) 2016 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; see the file COPYING.LIB.  If
+   not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef _MALLOC_HOOKS_H
+#define _MALLOC_HOOKS_H
+
+/* These hooks are no longer part of the public API and are poisoned
+   in <stdc-predef.h>.  Their names here reflect the command-line
+   mapping which is used inside glibc to get past the poisoning.  */
+void (*old__malloc_initialize_hook) (void);
+
+#endif  /* _MALLOC_HOOKS_H */