target/riscv: Adjust scalar reg in vector with XLEN

target/riscv: Adjust scalar reg in vector with XLEN

When sew <= 32bits, not need to extend scalar reg.
When sew > 32bits, if xlen is less that sew, we should sign extend
the scalar register, except explicitly specified by the spec.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220120122050.41546-21-zhiwei_liu@c-sky.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>