RISC-V: Minimal support of scalar crypto extension.

Minimal support of scalar crypto extension, add "k" in the
riscv_supported_std_ext, to make the order check right with
"zk" behind "zb".

bfd/
	* elfxx-riscv.c (riscv_implicit_subsets): Added implicit
	rules for zk* extensions.
	(riscv_supported_std_ext): Added entry for k.
	(riscv_supported_std_z_ext): Added entries for zk*.