AArch64 Memory issues

When running the kernel in qemu with the  hermit-loader and supplying around 4 GiB of Memory, then the kernel crashes when reserving the heap memory with an unwrap of an error value:

```
[0][PANIC] panicked at src/mm/mod.rs:155:18:
called `Result::unwrap()` on an `Err` value: AllocError
```