Linux: consolidate readlink implementation

Use readlink syscall if defined, otherwise readlinkat.

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

sysdeps/unix/sysv/linux/readlink.c

@@ -0,0 +1,35 @@
+/* Read value of a symbolic link.  Linux version.
+   Copyright (C) 2011-2022 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 <unistd.h>
+#include <fcntl.h>
+#include <sysdep.h>
+
+/* Read the contents of the symbolic link PATH into no more than
+   LEN bytes of BUF.  The contents are not null-terminated.
+   Returns the number of characters read, or -1 for errors.  */
+ssize_t
+__readlink (const char *path, char *buf, size_t len)
+{
+#ifdef __NR_readlink
+  return INLINE_SYSCALL_CALL (readlink, path, buf, len);
+#else
+  return INLINE_SYSCALL_CALL (readlinkat, AT_FDCWD, path, buf, len);
+#endif
+}
+weak_alias (__readlink, readlink)