gh-116968: Reimplement Tier 2 counters (#117144)

Introduce a unified 16-bit backoff counter type (``_Py_BackoffCounter``),
shared between the Tier 1 adaptive specializer and the Tier 2 optimizer. The
API used for adaptive specialization counters is changed but the behavior is
(supposed to be) identical.

The behavior of the Tier 2 counters is changed:
- There are no longer dynamic thresholds (we never varied these).
- All counters now use the same exponential backoff.
- The counter for ``JUMP_BACKWARD`` starts counting down from 16.
- The ``temperature`` in side exits starts counting down from 64.

Modules/_testinternalcapi.c

@@ -10,6 +10,7 @@
 #undef NDEBUG
 
 #include "Python.h"
+#include "pycore_backoff.h"       // JUMP_BACKWARD_INITIAL_VALUE
 #include "pycore_bitutils.h"      // _Py_bswap32()
 #include "pycore_bytesobject.h"   // _PyBytes_Find()
 #include "pycore_ceval.h"         // _PyEval_AddPendingCall()
@@ -1819,6 +1820,11 @@ module_exec(PyObject *module)
         return 1;
     }
 
+    if (PyModule_Add(module, "TIER2_THRESHOLD",
+                        PyLong_FromLong(JUMP_BACKWARD_INITIAL_VALUE)) < 0) {
+        return 1;
+    }
+
     return 0;
 }