[Executorch][Kernels] Build optimized lib with -O2 (#6586)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6517

Dont rely on top level config for optimization and rather set our own for optimized kernels
ghstack-source-id: 251021623
@exported-using-ghexport

Differential Revision: [D64910575](https://our.internmc.facebook.com/intern/diff/D64910575/)

Co-authored-by: Kimish Patel <kimishpatel@fb.com>