[mypyc] Fix crash on accessing StopAsyncIteration (#21406)
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](https://github.com/python/cpython/blob/main/Include/pyerrors.h#L80)
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 *`.
