[RISCV] Implement writeback and signedness for Core-V SIMD extension

Created by: melonedo

In #27, instructions were added regardless of their semantics in the form of a "pure" function, i.e. rd = f(rs1, rs2). However, many of them actually read rd, i.e. rd = f(rd, rs1, rs2). Also, #27 did not take into account the signedness of the immediates, which is also fixed in this PR.