Use portable atomics crate

[kraj]

This extends building thing-provisioning-tools for 32bit architectures without 64bit atomics support e.g. armv5, mips, ppc32 and riscv32. More info on portable-atomics [1]

Fixes

error[E0432]: unresolved import std::sync::atomic::AtomicU64 │ 232 --> src/cache/writeback.rs:6:25 │ 234 | use std::sync::atomic::{AtomicU64, Ordering}; │ 235 | ^^^^^^^^^

[1] https://crates.io/crates/portable-atomic