libio: Do not autogenerate stdio_lim.h

Instead define the required fields in system dependend files.  The only
system dependent definition is FILENAME_MAX, which should match POSIX
PATH_MAX, and it is obtained from either kernel UAPI or mach headers.
Currently set pre-defined value from current kernels.

It avoids a circular dependendy when including stdio.h in
gen-as-const-headers files.

Checked on x86_64-linux-gnu and i686-linux-gnu
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

sysdeps/unix/sysv/linux/bits/stdio_lim.h

@@ -0,0 +1,28 @@
+/* System specific stdio.h definitions.  Linux version.
+   Copyright (C) 2023 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/>.  */
+
+#ifndef _BITS_STDIO_LIM_H
+#define _BITS_STDIO_LIM_H 1
+
+#ifndef _STDIO_H
+# error "Never include <bits/stdio_lim.h> directly; use <stdio.h> instead."
+#endif
+
+#define FILENAME_MAX 4096
+
+#endif /* bits/stdio_lim.h */