annotate the missing private functions in `numpy.linalg._linalg`

it's used by `array-api-strict` in https://github.com/crusaderky/array-api-strict/blob/a8f567a184ceb2f94c6160695b63dd9c5f2454d7/array_api_strict/_linalg.py#L316-L326 and `array-api-compat` in https://github.com/data-apis/array-api-compat/blob/16978e63b744185bc4ced4677c822468e6ddebd2/array_api_compat/numpy/linalg.py#L46-L49

