cpu: Introduce CPUNegativeOffsetState

Nothing in there so far, but all of the plumbing done
within the target ArchCPU state.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/mips/cpu.h

@@ -1068,6 +1068,7 @@ struct MIPSCPU {
     CPUState parent_obj;
     /*< public >*/
 
+    CPUNegativeOffsetState neg;
     CPUMIPSState env;
 };