target/riscv: fix ctzw behavior

target/riscv: fix ctzw behavior

According to spec, ctzw should work with 32-bit register, not 64.

For example, previous implementation returns 33 for (1<<33) input
when the new one returns 32.

Signed-off-by: Vladimir Isaev <vladimir.isaev@syntacore.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230204082312.43557-1-vladimir.isaev@syntacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>