Avoid importing alloc::sync on platforms that do not have it

[AverageHelper]

While trying to test another crate (which depends on maud) under no_std conditions, I found that this won't compile as-is on my go-to baseline target for that (usually thumbv6m-none-eabi), due to a lack of atomic pointer support. (thumbv7m-none-eabi seems to work fine however.)

Since maud only seems to import Arc to add a Render impl, I suppose it wouldn't affect anything to simply move that impl block behind an appropriate cfg attribute so it won't be included when building for affected platforms. This PR does just that.

In terms of tests, I could add a CI step to try building against thumbv6m-none-eabi if you'd like, but I figured I'd ask on that one, since no_std support seems to work well here otherwise.