target/riscv: Remove the hardcoded RVXLEN macro

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: a07bc0c6dc4958681b4f93cbc5d0acc31ed3344a.1619234854.git.alistair.francis@wdc.com