Update syscall-names.list for Linux 4.20.

This patch updates sysdeps/unix/sysv/linux/syscall-names.list for
Linux 4.20.  Although there are no new syscalls, the
riscv_flush_icache syscall has moved to asm/unistd.h (previously in
asm/syscalls.h) and so now needs to be added to the list.

Tested with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/syscall-names.list: Update kernel
	version to 4.20.
	(riscv_flush_icache): New syscall.