Fix the upstream alloc_system crate instead of using this one

[FenrirWolf]

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.