Integrate mmap model from CBMC

[feliperodri]

Currently, we have harnesses blocked by unsupported libc libraries. Current blockers:

• mmap;
• sysconf;

We should rely on CBMC C Library models while make sure that customers can create their own stubs of these models in Kani as well.

[feliperodri]

We are unable to fully support mmap due to the following issue diffblue/cbmc#7773.

[celinval]

I think we need a better mechanism to import these functions to Kani. In the meantime, we might want to keep doing this by demand.

[roypat]

For what its worth with the mmap one, firecracker only uses MAP_ANONYMOUS without MAP_FIXED, which if I read CBMC code correctly wont run into the deprecation issue you linked.