opcodes/riscv-dis.c: Use bool type whenever possible

opcodes/riscv-dis.c: Use bool type whenever possible

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

This commit replaces uses of int with bool whenever possible.

opcodes/ChangeLog:

	* riscv-dis.c (no_aliases) Change type to bool.
	(set_default_riscv_dis_options): Use boolean.
	(parse_riscv_dis_option_without_args): Likewise.
	(riscv_disassemble_insn): Use boolean keywords.