target/riscv: Replace is_32bit with get_xl/get_xlen

target/riscv: Replace is_32bit with get_xl/get_xlen

In preparation for RV128, replace a simple predicate
with a more versatile test.

Reviewed-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20211020031709.359469-9-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>