Enable Ethos-U85 support in Vela (#5002)

Summary:
Switch pinned version of Vela with Ethos-U85 support. Adapt the compiler arguments and add a compile_spec to support targetting Ethos-U85.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5002

Reviewed By: mergennachin

Differential Revision: D62186731

Pulled By: digantdesai

fbshipit-source-id: 3716606bea665fa7472f1159275183bc088697e4