Update struct signalfd_siginfo from Linux 4.18.

This patch updates struct signalfd_siginfo in sys/signalfd.h with new
members from Linux 4.18 (plus ssi_addr_lsb, added to the kernel in
2.6.37 without being added to sys/signalfd.h at that time).  The
__pad2 member name follows the kernel and the existing __pad name.

Tested for x86_64.

	* sysdeps/unix/sysv/linux/sys/signalfd.h (struct
	signalfd_siginfo): Add ssi_addr_lsb, ssi_syscall, ssi_call_addr
	and ssi_arch members.