signal: Move sys_siglist to a compat symbol

The symbol was deprecated by strsignal and its usage imposes issues
such as copy relocations.

Its internal name is changed to __sys_siglist and __sys_sigabbrev to
avoid static linking usage.  The compat code is also refactored, since
both Linux and Hurd usage the same strategy: export the same array with
different object sizes.

The libSegfault change avoids calling strsignal on the SIGFAULT signal
handler (the current usage is already sketchy, adding a call that
potentially issue locale internal function is even sketchier).

Checked on x86_64-linux-gnu and i686-linux-gnu. I also run a check-abi
on all affected platforms.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

sysdeps/unix/sysv/linux/siglist-compat.c

@@ -0,0 +1,31 @@
+/* Compatibility signal numbers and their names symbols.  Linux version.
+   Copyright (C) 1997-2020 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 <siglist-compat.h>
+
+#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
+DEFINE_COMPAT_SIGLIST (32, GLIBC_2_0)
+#endif
+
+#if SHLIB_COMPAT (libc, GLIBC_2_1, GLIBC_2_3_3)
+DEFINE_COMPAT_SIGLIST (64, GLIBC_2_1)
+#endif
+
+#if SHLIB_COMPAT (libc, GLIBC_2_3_3, GLIBC_2_32)
+DEFINE_COMPAT_SIGLIST (NSIG, GLIBC_2_3_3)
+#endif