target/riscv: Create xl field in env

target/riscv: Create xl field in env

Current xlen has been used in helper functions and many other places.
The computation of current xlen is not so trivial, so that we should
recompute it as little as possible.

Fortunately, xlen only changes in very seldom cases, such as exception,
misa write, mstatus write, cpu reset, migration load. So that we can only
recompute xlen in this places and cache it into CPURISCVState.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220120122050.41546-6-zhiwei_liu@c-sky.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>