Remove deprecated atomic-polyfill crate for portable-atomic

[BjornTheProgrammer]

This prevents a warning like the following from appearing when running cargo audit -q in another project.

➜  burn git:(embedded-fix) ✗ cargo audit -q --color always
Crate:     atomic-polyfill
Version:   1.0.3
Warning:   unmaintained
Title:     atomic-polyfill is unmaintained
Date:      2023-07-11
ID:        RUSTSEC-2023-0089
URL:       https://rustsec.org/advisories/RUSTSEC-2023-0089
Dependency tree:
atomic-polyfill 1.0.3
└── embassy-rp 0.8.0

[BjornTheProgrammer]

I would appreciate some help on this to figure out how to get the builds working properly for the tests.