RISC-V: Print comma and tabs as the "text" style

On the RISC-V disassembler, some separators have non-text style when
printed with another word with another style.

This commit splits those, making sure that those comma and tabs are printed
with the "text" style.

opcodes/ChangeLog:

	* riscv-dis.c (print_insn_args): Split and print the comma as
	text.  (riscv_disassemble_insn): Split and print tabs as text.
	(riscv_disassemble_data): Likewise.