[mypyc] Fix crash on accessing StopAsyncIteration

[p-sawicki]

Using StopAsyncIteration in raise expressions or as the caught type in except blocks crashes at runtime because the generated code loads the address of the PyExc_StopAsyncIteration variable and casts it to PyObject *. The variable itself is already a PyObject * so its address being interpreted as PyObject * is incorrect.

To fix, add a special case to use LoadGlobal instead of LoadAddress for builtin type variables that are already PyObject *.