Regression fixes and tidyup for Arm backend (#2372)

Summary:
Fixes for a few things:

- Move C++ std to 17 to enable changes for https://github.com/pytorch/executorch/issues/1733 on Arm builds
- fix after https://github.com/pytorch/executorch/issues/2307 regressed save_pte_program on aot_arm_compiler
- add aot_arm_compiler support for examples/models (they don't all compile but can be consumed)
- Move the Vela pin in anticipation of MNV2 tosa fixes in https://github.com/pytorch/executorch/issues/2371

Pull Request resolved: https://github.com/pytorch/executorch/pull/2372

Reviewed By: mcr229

Differential Revision: D54817009

Pulled By: digantdesai

fbshipit-source-id: 5ca3c7a03e8ae9f6a784335d53654b8db05a3cb8