stdio: Remove the usage of $(fno-unit-at-a-time) for siglist.c

The siglist.c is built with -fno-toplevel-reorder to avoid compiler
to reorder the compat assembly directives due an assembler
issue [1] (fixed on 2.39).

This patch removes the compiler flags by split the compat symbol
generation in two phases.  First the __sys_siglist and __sys_sigabbrev
without any compat symbol directive is preprocessed to generate an
assembly source code.  This generate assembly is then used as input
on a platform agnostic siglist.S which then creates the compat
definitions.  This prevents compiler to move any compat directive
prior the _sys_errlist definition itself.

Checked on a make check run-built-tests=no on all affected ABIs.

Reviewed-by: Fangrui Song <maskray@google.com>

stdio-common/siglist-gen.c

@@ -0,0 +1,35 @@
+/* Define list of all signal numbers and their names.
+   Copyright (C) 1997-2022 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/>.  */
+
+#include <stddef.h>
+#include <signal.h>
+#include <libintl.h>
+
+const char *const __sys_siglist[NSIG] =
+{
+#define init_sig(sig, abbrev, desc)   [sig] = desc,
+#include <siglist.h>
+#undef init_sig
+};
+
+const char *const __sys_sigabbrev[NSIG] =
+{
+#define init_sig(sig, abbrev, desc)   [sig] = abbrev,
+#include <siglist.h>
+#undef init_sig
+};