RISC-V: Emit "i" suffix for instructions with immediate operands

This changes makes GCC asm output use instruction names that are
consistent with the RISC-V ISA manual.

- Add 'i' letter to riscv_print_operand that prints an i
  character if the operand is not a register.
- Update riscv.md to add %i2 to instructions with register
  register and register immediate variants.