Refactor extract_segments out of serialize_pte_binary (#1353)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1353

`serialize_pte_binary` is getting long; refactor out segment extraction into a separate function.

Reviewed By: dbort

Differential Revision: D51817537

fbshipit-source-id: be1cace120b504a83f0ac967d9145980a20a8ed9