target/openrisc: Use cpu_unwind_state_data for mfspr

Since we do not plan to exit, use cpu_unwind_state_data
and extract exactly the data requested.

This is a bug fix, in that we no longer clobber dflag.

Consider:

        l.j       L2         // branch
        l.mfspr   r1, ppc    // delay

L1:     boom
L2:     l.lwa     r3, (r4)

Here, dflag would be set by cpu_restore_state (because that is the current
state of the cpu), but but not cleared by tb_stop on exiting the TB
(because DisasContext has recorded the current value as zero).

The next TB begins at L2 with dflag incorrectly set.  If the load has a
tlb miss, then the exception will be delivered as per a delay slot:
with DSX set in the status register and PC decremented (delay slots
restart by re-executing the branch). This will cause the return from
interrupt to go to L1, and boom!

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

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