Update syscall-names.list for Linux 5.3.

This patch updates syscall-names.list for Linux 5.3, adding two new
syscalls.

Tested with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/syscall-names.list: Update kernel
	version to 5.3.
	(clone3): New syscall.
	(pidfd_open): Likewise.