Fix the upstream `alloc_system` crate instead of using this one

The only reason we use this crate instead of the in-tree version of `alloc_system` is because devkitARM doesn't have `posix_memalign`, so we have to make sure that `memalign` is chosen instead. I suppose we could just throw a PR at rust-lang and see what happens, but I'm not sure how much they care to make changes to internal implementation details for the benefit of out-of-tree consumers.