RISC-V: Fix the order checking for Z* extension.

We have to check the first char of the Z* extensions, to make sure that
they follow the order of the standard extensions.  But we can not have
the testcases for this patch, since we only support the zicsr and zifencei
so far, both of them are the sub extensions of i.

	bfd/
	* elfxx-riscv.c (riscv_parse_prefixed_ext): Use riscv_compare_subsets
	to check the Z* extensions' order.