Update on "[ExecuTorch] Arm Ethos:Make get_compile_spec() configurable"

Allow users to generate different compile specs based on the Args.
Preserve the behavior.
No change in the default spec returned.

Differential Revision: [D70351486](https://our.internmc.facebook.com/intern/diff/D70351486/)

[ghstack-poisoned]

Author: digantdesai