linux: Consolidate Linux open_by_handle_at syscall

linux: Consolidate Linux open_by_handle_at syscall

This patch consolidates the open_by_handle_at Linux syscall generation on
sysdeps/unix/sysv/linux/open_by_handle_at.c.  It basically removes it from
architectures auto-generation list.

Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
arch64-linux-gnu, arm-linux-gnueabihf, powerpc64le-linux-gnu,
sparc64-linux-gnu, and sparcv9-linux-gnu.

	* sysdeps/unix/sysv/linux/Makefile (sysdep_routines): Add
	open_by_handle_at.
	(CFLAGS-open_by_handle_at.c): New flag.
	* sysdeps/unix/sysv/linux/open_by_handle_at.c: New file.
	* sysdeps/unix/sysv/linux/syscalls.list (open_by_handle_at): New
	file.
	* misc/Makefile (CFLAGS-open_by_handle_at.c): New rule.