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>

include/exec/cpu-defs.h

@@ -227,4 +227,12 @@ typedef struct CPUTLB {
 
 #endif  /* !CONFIG_USER_ONLY && CONFIG_TCG */
 
+/*
+ * This structure must be placed in ArchCPU immedately
+ * before CPUArchState, as a field named "neg".
+ */
+typedef struct CPUNegativeOffsetState {
+    /* Empty */
+} CPUNegativeOffsetState;
+
 #endif