Arm backend: Change run.sh to let cmake decide number of parallel jobs (#5813)

Summary:
Instead of specifying number of parallel jobs let cmake decide for it self. This make this script work better on macOS and might speed up builds as more parallel jobs are used.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5813

Reviewed By: mergennachin

Differential Revision: D63809604

Pulled By: digantdesai

fbshipit-source-id: 905fde94a270cd2cacb1257857962ad507a4cbc5