RISC-V: Fix incorrect disassembly for addiw

RISC-V: Fix incorrect disassembly for addiw

This fixes a bug in the disassembler constraints used
to lift instructions into pseudo-instructions, whereby
addiw instructions are always lifted to sext.w instead
of just lifting addiw with a zero immediate.

An associated fix has been made to the metadata used to
machine generate the disseasembler:

https://github.com/michaeljclark/riscv-meta/
commit/4a6b2f3898430768acfe201405224d2ea31e1477

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>