gh-155628: Use a relaxed add for the asyncio task name counter

The counter only generates unique default Task names; the add is its
sole access in the free-threaded build, so only the uniqueness of each
returned value matters, which atomicity alone guarantees.

Author: kddnewton

Misc/NEWS.d/next/C_API/2026-08-20-15-21-46.gh-issue-155628.9QggKx.rst

@@ -0,0 +1,5 @@
+Add relaxed variants of the _Py_atomic_add functions. The existing add
+operations are sequentially consistent, which is stronger (and on ARM,
+measurably more expensive) than necessary for uses like statistics counters
+and unique ID allocation, where the add must be atomic but does not need to
+order surrounding memory accesses.

Modules/_asynciomodule.c

@@ -2342,7 +2342,7 @@ _asyncio_Task___init___impl(TaskObj *self, PyObject *coro, PyObject *loop,
         // store the task counter as PyLong in the name
         // for deferred formatting in get_name
 #ifdef Py_GIL_DISABLED
-        unsigned long long counter = _Py_atomic_add_uint64(&state->task_name_counter, 1) + 1;
+        unsigned long long counter = _Py_atomic_add_uint64_relaxed(&state->task_name_counter, 1) + 1;
 #else
         unsigned long long counter = ++state->task_name_counter;
 #endif