[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 *.

[JukkaL]

Now the name doesn't quite match the intent -- it either loads the address or the value of the name. Instead of adding a bool flag to the control for address/value, I think it would be better to add a new dictionary for something like load_global_op(<fullname>, <type>, <c_name>). This way the pre-existing buildin_names dictionary wouldn't have to be touched at all, I think.