Address various warnings as errors (#8581)

Address various warnings as errors (#8581)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/8581

Some projects uses more restrictive build options than currently used in ET CI.

This means we encountered a number of errors when enabling for a microcontroller.

Reviewed By: digantdesai, swolchok

Differential Revision: D69139962