RISC-V: Add misa to DisasContext

gen methods should access state from DisasContext. Add misa
field to the DisasContext struct and remove CPURISCVState
argument from all gen methods.

Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>