gh-112175: Add eval_breaker to PyThreadState (#115194)

This change adds an `eval_breaker` field to `PyThreadState`. The primary
motivation is for performance in free-threaded builds: with thread-local eval
breakers, we can stop a specific thread (e.g., for an async exception) without
interrupting other threads.

The source of truth for the global instrumentation version is stored in the
`instrumentation_version` field in PyInterpreterState. Threads usually read the
version from their local `eval_breaker`, where it continues to be colocated
with the eval breaker bits.

Python/brc.c

@@ -94,7 +94,7 @@ _Py_brc_queue_object(PyObject *ob)
     }
 
     // Notify owning thread
-    _Py_set_eval_breaker_bit(interp, _PY_EVAL_EXPLICIT_MERGE_BIT, 1);
+    _Py_set_eval_breaker_bit(&tstate->base, _PY_EVAL_EXPLICIT_MERGE_BIT);
 
     PyMutex_Unlock(&bucket->mutex);
 }