target/riscv: Split misa.mxl and misa.ext

target/riscv: Split misa.mxl and misa.ext

The hw representation of misa.mxl is at the high bits of the
misa csr.  Representing this in the same way inside QEMU
results in overly complex code trying to check that field.

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: 20211020031709.359469-4-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>