Does not compile on thumbv6m-none-eabi

[CryZe]

The crate fails compiling on thumbv6m-none-eabi. In my case it's included as an indirect dependency of cosmic-text.

error[E0433]: failed to resolve: could not find `sync` in `alloc`
Error:    --> /home/runner/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/ouroboros-0.15.6/src/lib.rs:372:46
    |
372 |     std_type_check!(is_std_arc_type T alloc::sync::Arc<T>);
    |                                              ^^^^ could not find `sync` in `alloc`

The sync module only exists if the following cfg holds #[cfg(target_has_atomic = "ptr")]. This cfg is available in stable Rust.