Use portable-atomic on platforms without u64 atomics

[linkmauve]

This has been tested on powerpc-unknown-linux-gnu, which supports atomics only up to u32. Without this commit, building kira would fail with:

error[E0432]: unresolved import `std::sync::atomic::AtomicU64`
 --> crates/kira/src/backend/cpal/desktop/stream_manager.rs:6:24
  |
6 |         atomic::{AtomicBool, AtomicU64, Ordering},
  |                              ^^^^^^^^^
  |                              |
  |                              no `AtomicU64` in `sync::atomic`
  |                              help: a similar name exists in the module: `AtomicU32`

Edit: This also depends on not-fl3/quad-rand#23 in order to make the whole testsuite pass, but can be merged independently.

[linkmauve]

I just rebased that PR on top of latest main, nothing much was changed but there was a merge conflict.

[tesselode]

Which test isn't passing? I'm unclear on what this has to do with quad-rand.

[linkmauve]

quad-rand just has the exact same issue as kira, it also uses AtomicU64 for its prng, which isn’t available on PowerPC so it also doesn’t build there.

This PR is the only one required to make kira work on Linux on the Wii, but I’ve also sent a PR to quad-rand to also make kira’s testsuite compile there.