Remove 64-bit atomics in main crate

[kay-lambdadelta]

This allows the base egui crate to run on platforms without 64 bit atomics

• Addresses Use AtomicU32 in place of AtomicU64 #7692, but does not address the egui_extras crate

• I have followed the instructions in the PR template

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/8037-remove-64-bit-atomics
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

View snapshot changes at kitdiff

[lucasmerlin]

Thanks!
Maybe we can also forbid the use of AtomicU64 with clippy so we don't regress on this in the future?

[kay-lambdadelta]

Thanks! Maybe we can also forbid the use of AtomicU64 with clippy so we don't regress on this in the future?

Something this would work I believe.

disallowed-types = [
    { path = "std::sync::atomic::AtomicU64", reason = "AtomicU64 is not supported on many common platforms, use AtomicU32 or AtomicUsize instead" }
]