Use `core::mem::offset_of!()` macro (after moving to Rust 1.77.0)

[zulinx86]

Derived from discussion here: https://github.com/firecracker-microvm/firecracker/pull/4492#discussion_r1526045051 After we move to Rust 1.77.0 (the current latest is 1.76.0 at the time of writing), core::mem::offset_of!() macro seems to become stable finally!! Currently, we have our own macro called offset__of!(), but thanks to the stabilization, we might be able to remove it.