Add _PY_FASTCALL_SMALL_STACK constant

Issue #28870: Add a new _PY_FASTCALL_SMALL_STACK constant, size of "small
stacks" allocated on the C stack to pass positional arguments to
_PyObject_FastCall().

_PyObject_Call_Prepend() now uses a small stack of 5 arguments (40 bytes)
instead of 8 (64 bytes), since it is modified to use _PY_FASTCALL_SMALL_STACK.

Objects/abstract.c

@@ -2337,7 +2337,7 @@ PyObject *
 _PyObject_Call_Prepend(PyObject *callable,
                        PyObject *obj, PyObject *args, PyObject *kwargs)
 {
-    PyObject *small_stack[8];
+    PyObject *small_stack[_PY_FASTCALL_SMALL_STACK];
     PyObject **stack;
     Py_ssize_t argcount;
     PyObject *result;
@@ -2523,7 +2523,7 @@ static PyObject *
 _PyObject_CallFunctionVa(PyObject *callable, const char *format,
                          va_list va, int is_size_t)
 {
-    PyObject* small_stack[5];
+    PyObject* small_stack[_PY_FASTCALL_SMALL_STACK];
     const Py_ssize_t small_stack_len = Py_ARRAY_LENGTH(small_stack);
     PyObject **stack;
     Py_ssize_t nargs, i;
@@ -2704,7 +2704,7 @@ _PyObject_CallMethodId_SizeT(PyObject *obj, _Py_Identifier *name,
 PyObject *
 _PyObject_VaCallFunctionObjArgs(PyObject *callable, va_list vargs)
 {
-    PyObject *small_stack[5];
+    PyObject *small_stack[_PY_FASTCALL_SMALL_STACK];
     PyObject **stack;
     Py_ssize_t nargs;
     PyObject *result;