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.

Python/bltinmodule.c

@@ -1186,7 +1186,7 @@ map_traverse(mapobject *lz, visitproc visit, void *arg)
 static PyObject *
 map_next(mapobject *lz)
 {
-    PyObject *small_stack[5];
+    PyObject *small_stack[_PY_FASTCALL_SMALL_STACK];
     PyObject **stack;
     Py_ssize_t niters, nargs, i;
     PyObject *result = NULL;