Functions for Integer sign and unsign are missing

[ptprasanna]

https://github.com/riscv-software-src/riscv-isac/blob/9dc9503c9cce79d8c93d8f4955111f2f6d1ad9c7/riscv_isac/InstructionObject.py#L285

For rs3 the functions has to be defined with respect to sign and unsigned values.