Arm CI fix (#2771)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2771

- buck issue with inotify limit. Default is 8K. 24K seems to work. Setting it to 1M. Didn't investigate further.
- top CMake always building cpuinfo and threadpool. Added an option to turn it off with ARM_BAREMETAL
- arm example building w/o ARM_BAREMETAL. Added that.

Reviewed By: shoumikhin, Gasoonjia, mikekgfb

Differential Revision: D55538019

fbshipit-source-id: 1a22504d157e47a1052e9b49ca28d56eb5eb22c5