build: Select heapless-crate with features for GBA support

[Teufelchen1]

Hey 🦋

This change is needed to compile RIOT/examples/rust-hello-world for the GBA.
I am aware that this is not the right way to do it but I am not yet proficient with Rust yet, so... please help! :)

The portable-atomic documentation even mentions the GBA.
The version bump of heapless is needed as the feature is not yet present in 0.7.x.

[chrysn]

Unfortunately heapless is part of our public API, but judging from the number of issues/PRs with the wait-for-breaking label, now is probably the time for riot-wrappers 0.9!

[chrysn]

I'm a bit uneasy about adding the assertion on a single core (simply because while RIOT can't do multicore now, we'd miss that when changing that), but I think there's a better way: portable-atomic can also work with critical-section, and we have support for that – and given that users of our irq_off() rely on all the world to stop, I think it's safe to say that whatever happens, irq_off will stop interrupts as well as other cores.

[chrysn]

Merged into the v09 branch. We can still provide a better atomic implementation later, but for now single-core is correct, and it was the API breakage that held it up.

[chrysn]

I had to revert the merge because the single-CPU feature appears to break native.

Let's keep this open across the v09 merge – the underlying issue of updating heapless is resolved, a workaround of enabling the heapless feature in some other crate should work for the time being, and let's look on for how to do this in a way that works on all boards.