Automatically enable portable-atomic when required

[bushrat011899]

Objective

• Contributes to Tracking Issue: MVP no_std Bevy #15460
• Reduce quantity and complexity of feature gates across Bevy

Solution

• Used target_has_atomic configuration variable to automatically detect impartial atomic support and automatically switch to portable-atomic over the standard library on an as-required basis.

Testing

• CI

Notes

To explain the technique employed here, consider getting Arc either from alloc::sync or portable-atomic-util. First, we can inspect the alloc crate to see that you only have access to Arc if target_has_atomic = "ptr". We add a target dependency for this particular configuration inverted:

[target.'cfg(not(target_has_atomic = "ptr"))'.dependencies]
portable-atomic-util = { version = "0.2.4", default-features = false }

This ensures we only have the dependency when it is needed, and it is entirely excluded from the dependency graph when it is not. Next, we adjust our configuration flags to instead of checking for feature = "portable-atomic" to instead check for target_has_atomic = "ptr":

// `alloc` feature flag hidden for brevity

#[cfg(not(target_has_atomic = "ptr"))]
use portable_atomic_util as arc;

#[cfg(target_has_atomic = "ptr")]
use alloc::sync as arc;

pub use arc::{Arc, Weak};

The benefits of this technique are three-fold:

1. For platforms without full atomic support, the functionality is enabled automatically.
2. For platforms with atomic support, the dependency is never included, even if a feature was enabled using --all-features (for example)
3. The portable-atomic feature no longer needs to virally spread to all user-facing crates, it's instead something handled within bevy_platform_support (with some extras where other dependencies also need their features enabled).

[bushrat011899]

Some notes to help reviewers.

[bushrat011899]

No longer required, since we can now guarantee that bevy_platform_support::sync::Arc will be alloc::sync::Arc if it exists.

[bushrat011899]

Since bevy_platform_support can now automatically detect the need for portable-atomic, user-facing crates no longer need to provide that feature.

[bushrat011899]

Since bevy_ecs has a dependency which itself has a portable-atomic feature, we replicate the hack here to just automatically enable its feature on required targets.

[BenjaminBrienen]

The trick could be upstreamed at some point

[bushrat011899]

Since we only need portable-atomic-util for Arc, and Arc is only available when target_has_atomic = "ptr", we can simplify its target dependency a little.

[bushrat011899]

Essentially, "If any atomic feature isn't available, pessimistically include portable-atomic"

[bushrat011899]

Just a little tidy-up while I'm here. I'm confident the pointer-cast is valid and it avoids a second allocation.