Override std::ptr::align_offset

[tautschnig]

This hook intercepts calls to std::ptr::align_offset<T> as CBMC's memory model has no concept of alignment of allocations, so we would have to non-deterministically choose an alignment of the base pointer, add the pointer's offset to it, and then do the math that is done in library/core/src/ptr/mod.rs. Instead, we choose to always return either 0 when the pointer points to the beginning of an object (and, therefore, is necessarily aligned), or usize::MAX, per align_offset's documentation, which states: "It is permissible for the implementation to always return usize::MAX. Only your algorithm’s performance can depend on getting a usable offset here, not its correctness."

Fixes: #2363

[tautschnig]

Perhaps @karkhaz:

1. The alleged performance regression (https://github.com/model-checking/kani/actions/runs/4753944159/jobs/8446153448?pr=2396) is the same as previously reported upon a merge into main: https://github.com/model-checking/kani/actions/runs/4747428362/jobs/8432323295.
2. Would benchcomp also tell us about any performance improvements?

[zhassan-aws]

Is there any potential unsoundness with this change?

[celinval]

I don't think this implementation is sound. The code should panic if the input is not a power of two.
For example, the following code panic in Rust:

use std::mem::align_of;

fn main() {
  let x = 10;
  let ptr = &x as *const i32;
  let _ = ptr.align_offset(5);
}

Requested changes have been implemented.