opcodes/riscv: add styling support to print_reg_list

opcodes/riscv: add styling support to print_reg_list

I noticed that some unstyled output had crept into the risc-v
disassembler in this commit:

  commit 9132c8152b899a1683bc886f8ba76bedadb48aa1
  Date:   Tue Feb 27 11:48:11 2024 +0800

      RISC-V: Support Zcmp push/pop instructions.

this commit adds styling support.  The risc-v disassembler is now once
again, fully styled.