gen_custom_ops_aot_lib should always use rtti and exceptions (#1096)

Summary:
It uses PyTorch deps, and it needs -frtti -fexceptions.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1096

Test Plan: set(CMAKE_BUILD_TYPE Release), and cmake build

Reviewed By: larryliu0820

Differential Revision: D50677250

Pulled By: kirklandsign

fbshipit-source-id: 1260b7c12e4707bfbb99e4836056e3d67574727e