Skip assertion in gc/default.c when multi-Ractor

The counter for total allocated objects may not be accurate when there are
multiple Ractors since it is not atomic so there could be race conditions
when it is incremented.

gc/default.c

@@ -2637,8 +2637,9 @@ newobj_alloc(rb_objspace_t *objspace, rb_ractor_newobj_cache_t *cache, size_t si
 
     rb_size_pool_t *size_pool = &size_pools[size_pool_idx];
     size_pool->total_allocated_objects++;
-    GC_ASSERT(SIZE_POOL_EDEN_HEAP(size_pool)->total_slots + SIZE_POOL_TOMB_HEAP(size_pool)->total_slots >=
-        (size_pool->total_allocated_objects - size_pool->total_freed_objects - size_pool->final_slots_count));
+    GC_ASSERT(rb_gc_multi_ractor_p() ||
+        SIZE_POOL_EDEN_HEAP(size_pool)->total_slots + SIZE_POOL_TOMB_HEAP(size_pool)->total_slots >=
+            (size_pool->total_allocated_objects - size_pool->total_freed_objects - size_pool->final_slots_count));
 
     return obj;
 }