ISACOV: Fix issue #1414

Created by: AyoubJalali

I replace the get_field_imm by get_data_imm method, and instead of passing constant argument of the imm field length (we can make some error in counting the field length),I used $bits to return automatically the length field. We can use the same logic for the rs1 & rs2, because in CVA6 rs1 & rs2 are 64bits length, but in CV32 are 32bits length. I change also the signed field, instead of passing directlly the value, i used the type definition (c_imm_is_signed).