disas/riscv: Use signed type for vector immediates

operand_vimm() sign-extends the 5-bit vector immediate field in bits 19:15, but returns uint32_t. This sends negative immediates through an unsigned type before they are assigned to rv_decode.imm. Return int32_t to match the signed value extracted by the helper. Signed-off-by: TANG Tiancheng <lyndra@linux.alibaba.com> Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com> Reviewed-by: Alistair Francis <alistair.francis@wdc.com> Message-ID: <20260703-b4-disas-xthead-fix-riscv-next-v4-3-84c566330bc7@linux.alibaba.com> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>