Move some *at definitions to syscalls.list (bug 14138).

Move some *at definitions to syscalls.list (bug 14138).

Continuing the move of syscall definitions to syscalls.list, where the
removal of support for old kernel versions has made this possible,
this patch moves definitions of various *at functions in
sysdeps/unix/sysv/linux/.

These particular moves are straightforward: there are no #includes of
these source files, no special architecture-specific versions, no
special symbol version handling and no aliases.  Each source file can
be replaced by a single line in sysdeps/unix/sysv/linux/syscalls.list.

Tested for x86_64.

	[BZ #14138]
	* sysdeps/unix/sysv/linux/syscalls.list (fchownat): New syscall.
	(linkat): Likewise.
	(mkdirat): Likewise.
	(readlinkat): Likewise.
	(renameat): Likewise.
	(symlinkat): Likewise.
	(unlinkat): Likewise.
	* sysdeps/unix/sysv/linux/fchownat.c: Remove file.
	* sysdeps/unix/sysv/linux/linkat.c: Likewise.
	* sysdeps/unix/sysv/linux/mkdirat.c: Likewise.
	* sysdeps/unix/sysv/linux/readlinkat.c: Likewise.
	* sysdeps/unix/sysv/linux/renameat.c: Likewise.
	* sysdeps/unix/sysv/linux/symlinkat.c: Likewise.
	* sysdeps/unix/sysv/linux/unlinkat.c: Likewise.