opcodes/riscv-dis.c: Tidying with comments/clarity

Before changing the core disassembler, we take care of minor code clarity
issues and improve readability.

First, we need to clarify the roles of variables and code portions.

opcodes/ChangeLog:

	* riscv-dis.c (xlen): Move before default_isa_spec. Add comment.
	(default_isa_spec, default_priv_spec): Add comment.
	(riscv_gpr_names, riscv_fpr_names): Likewise.
	(parse_riscv_dis_option_without_args): Likewise.
	(parse_riscv_dis_option, parse_riscv_dis_options): Likewise.
	(maybe_print_address): Likewise.
	(riscv_disassemble_insn): Fix comment about the Zfinx "extension".
	Add comment about the riscv_multi_subset_supports call.