Adjust proof tooling to support CBMC v6

[tautschnig]

Description

With CBMC v6, unwinding assertions are enabled by default, and object bits no longer need to be set at compile time. Update various build rules to use the latest template as provided with CBMC starter kit.

Test Steps

Tested in CI.

Checklist:

• [ ] I have tested my changes. No regression in existing tests.
• n/a I have modified and/or added unit-tests to cover the code changes in this Pull Request.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.