bpo-38249: Expand Py_UNREACHABLE() to __builtin_unreachable() in the release mode. (GH-16329)

Co-authored-by: Victor Stinner <vstinner@python.org>

Modules/_tracemalloc.c

@@ -712,7 +712,7 @@ tracemalloc_realloc(void *ctx, void *ptr, size_t new_size)
 
                The GIL and the table lock ensures that only one thread is
                allocating memory. */
-            Py_UNREACHABLE();
+            Py_FatalError("tracemalloc_realloc() failed to allocate a trace");
         }
         TABLES_UNLOCK();
     }