AArch64: Reimplement Paging

[mkroening]

Paging is similarly unsound to the old x86-64 implementation. Even Clippy throws an obvious error:

error: mutable borrow from immutable input(s)
   --> src/arch/aarch64/mm/paging.rs:515:52
    |
515 |     fn subtable<S: PageSize>(&self, page: Page<S>) -> &mut PageTable<L::SubtableLevel> {
    |                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
note: immutable borrow here
   --> src/arch/aarch64/mm/paging.rs:515:27
    |
515 |     fn subtable<S: PageSize>(&self, page: Page<S>) -> &mut PageTable<L::SubtableLevel> {
    |                              ^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mut_from_ref
    = note: `#[deny(clippy::mut_from_ref)]` on by default

Specialization is also problematic.

Unfortunately, there is no crate which provides a good paging implementation for AArch64.

This is the last part of #497.