py: Add independent config for debugging sentinel object values.

The new compile-time option is MICROPY_DEBUG_MP_OBJ_SENTINELS, disabled by
default.  This is to allow finer control of whether this debugging feature
is enabled or not (because, for example, this setting must be the same for
mpy-cross and the MicroPython main code when using native code generation).

py/emitnative.c

@@ -2094,12 +2094,12 @@ STATIC void emit_native_for_iter(emit_t *emit, mp_uint_t label) {
     emit_get_stack_pointer_to_reg_for_pop(emit, REG_ARG_1, MP_OBJ_ITER_BUF_NSLOTS);
     adjust_stack(emit, MP_OBJ_ITER_BUF_NSLOTS);
     emit_call(emit, MP_F_NATIVE_ITERNEXT);
-    #ifdef NDEBUG
-    MP_STATIC_ASSERT(MP_OBJ_STOP_ITERATION == 0);
-    ASM_JUMP_IF_REG_ZERO(emit->as, REG_RET, label, false);
-    #else
+    #if MICROPY_DEBUG_MP_OBJ_SENTINELS
     ASM_MOV_REG_IMM(emit->as, REG_TEMP1, (mp_uint_t)MP_OBJ_STOP_ITERATION);
     ASM_JUMP_IF_REG_EQ(emit->as, REG_RET, REG_TEMP1, label);
+    #else
+    MP_STATIC_ASSERT(MP_OBJ_STOP_ITERATION == 0);
+    ASM_JUMP_IF_REG_ZERO(emit->as, REG_RET, label, false);
     #endif
     emit_post_push_reg(emit, VTYPE_PYOBJ, REG_RET);
 }

py/mpconfig.h

@@ -414,6 +414,11 @@
 #define MICROPY_DEBUG_VERBOSE (0)
 #endif
 
+// Whether to enable debugging versions of MP_OBJ_NULL/STOP_ITERATION/SENTINEL
+#ifndef MICROPY_DEBUG_MP_OBJ_SENTINELS
+#define MICROPY_DEBUG_MP_OBJ_SENTINELS (0)
+#endif
+
 // Whether to enable a simple VM stack overflow check
 #ifndef MICROPY_DEBUG_VM_STACK_OVERFLOW
 #define MICROPY_DEBUG_VM_STACK_OVERFLOW (0)

py/obj.h

@@ -65,14 +65,14 @@ typedef struct _mp_obj_base_t mp_obj_base_t;
 // For debugging purposes they are all different.  For non-debug mode, we alias
 // as many as we can to MP_OBJ_NULL because it's cheaper to load/compare 0.
 
-#ifdef NDEBUG
-#define MP_OBJ_NULL             (MP_OBJ_FROM_PTR((void*)0))
-#define MP_OBJ_STOP_ITERATION   (MP_OBJ_FROM_PTR((void*)0))
-#define MP_OBJ_SENTINEL         (MP_OBJ_FROM_PTR((void*)4))
-#else
+#if MICROPY_DEBUG_MP_OBJ_SENTINELS
 #define MP_OBJ_NULL             (MP_OBJ_FROM_PTR((void*)0))
 #define MP_OBJ_STOP_ITERATION   (MP_OBJ_FROM_PTR((void*)4))
 #define MP_OBJ_SENTINEL         (MP_OBJ_FROM_PTR((void*)8))
+#else
+#define MP_OBJ_NULL             (MP_OBJ_FROM_PTR((void*)0))
+#define MP_OBJ_STOP_ITERATION   (MP_OBJ_FROM_PTR((void*)0))
+#define MP_OBJ_SENTINEL         (MP_OBJ_FROM_PTR((void*)4))
 #endif
 
 // These macros/inline functions operate on objects and depend on the