Consider renaming files named `mod.rs/x86_64.rs/aarch64.rs` to more unique filenames

Having multiple files with same name makes it annoying to search and find specific files.

For the `x86_64.rs/aarch64.rs` files (example `src/hyperlight_host/src/hypervisor/hyperlight_vm/x86_64.rs`) we could put them in a folder with that name instead (as we already do in certain places for example `src/hyperlight_host/src/hypervisor/regs/x86_64/special_regs.rs`).

For mod.rs file we can consider moving to the newer rust module convention which uses less mod.rs files.

I'm not saying all existing files must be renamed and are all bad, but I personally would like a bit more uniqueness

Start by inventorying the `mod.rs`, `x86_64.rs`, and `aarch64.rs` files, including `src/hyperlight_host/src/hypervisor/hyperlight_vm/x86_64.rs`, and compare them with `src/hyperlight_host/src/hypervisor/regs/x86_64/special_regs.rs`. Determine which files can use the newer Rust module convention and which architecture files should move into uniquely named directories. Done means the agreed renames are applied consistently without breaking the project checks.

索引モデルが issue の本文から書いたものです。