CBMC: Add 32-bit run

[hanno-becker]

As far as I understand, CBMC works with the configuration of native C implementation, such as the width of int and size_t, or void *. We currently only run CBMC on 64-bit platforms, which means that proofs would be valid for 64-bit environments only.

We should consider adding a CBMC run to CI which forces a 32-bit C implementation.