Fix portable-atomic on wgpu-hal

[bushrat011899]

Connections

• Follow-up to [wgpu-hal]: MVP no_std support #7599

Description

wgpu-hal has a portable-atomic feature, but alloc::sync::Arc is unconditionally used, causing a compilation failure on thumbv6m-none-eabi, as this target does not support atomic pointers. This can be almost trivially resolved by using portable-atomic-util, which provides an API-compatible Arc based on portable-atomic.

Note that while WGPU does not support thumbv6m-none-eabi (and it is not tested in CI), I am using it to represent targets without atomic pointers that the project may wish to support in the future.

Also note that this PR is branched from #7599, as no_std support is required to test the atomics failure condition. Once #7599 is merged, this PR will reduce to its most recent commit.

Finally, to ensure portable-atomic is kept out of the dependency graph, it is up to users to enable once_cell/portable-atomic on their own when required.

Testing

• Checked compilation on thumbv6m-none-eabi

Squash or Rebase?

Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry. N/A