Linux: Move timer_settime, __timer_settime64 from librt to libc

The symbols were moved using scripts/move-symbol-to-libc.py.

The way the ABI intransition is implemented is changed with this
commit: the implementation is now consolidated in one file with a
TIMER_T_WAS_INT_COMPAT check.

The shared librt is now empty, so this commit adds a placeholder
symbol at the base version, GLIBC_2.2, and potentially at the
GLIBC_2.3.3 version as well (the leftover from the int/timer_t ABI
transition).

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

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

@@ -0,0 +1,26 @@
+/* Placeholder definitions to pull in removed symbol versions.  Linux 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/>.  */
+
+#include <rt/librt-compat.c>
+#include <kernel-posix-timers.h>
+
+/* GLIBC_2.3.3 symbols were added for the int -> timer_t ABI transition.  */
+#if TIMER_T_WAS_INT_COMPAT
+compat_symbol (librt, __librt_version_placeholder_1,
+               __librt_version_placeholder, GLIBC_2_3_3);
+#endif