accel/tcg: Move CPUNegativeOffsetState into CPUState

Retain the separate structure to emphasize its importance.
Enforce CPUArchState always follows CPUState without padding.

Reviewed-by: Anton Johansson <anjo@rev.ng>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/mips/cpu.h

@@ -1213,10 +1213,10 @@ struct ArchCPU {
     CPUState parent_obj;
     /*< public >*/
 
+    CPUMIPSState env;
+
     Clock *clock;
     Clock *count_div; /* Divider for CP0_Count clock */
-    CPUNegativeOffsetState neg;
-    CPUMIPSState env;
 };