fix: boot on qemu x86_64

[nokosaaan]

Description

I fixed an issue where QEMU on x86_64 would not start properly.
This was resolved by clearly separating the BIOS and UEFI boot paths using the features option.
With the current configuration, the build process generated numerous warnings and errors,
preventing the program from starting successfully.
Specifically, the error error: unknown -Z flag specified: json-target-spec was being displayed.
The -Z flag is used to specify unstable, experimental features of the Rust compiler. It appears the issue arose because an outdated or incompatible version of Rust Nightly was used during the build of the Bootloader package, causing the flag to go unrecognized.
Possible solutions include:

• Using a newer Nightly build
• Updating the bootloader
• Using a known compatible version
  The nightly build is already the latest version, and the Bootloader is also kept up to date. Since we confirmed that it works without issues when using a compatible version, rolling back the version would be preferable; however, the cost of rework and the impact on existing code are significant, making this impractical.
  Therefore, since the BIOS processing is not expected to run in the current implementation, I isolated it in features. After confirming that it behaves the same as the compatible version, we decided to implement this fix.

In addition, a compile_error! guard was added to x86bootdisk/src/main.rs to produce a clear, actionable compiler error when the crate is built with --no-default-features and neither the bios nor uefi feature is selected, preventing confusing missing-impl errors in misconfigured builds.

Related links

https://github.com/rust-osdev/bootloader

How was this PR tested?

Please refer to attached files.
Compile logs are showed.
bootloader-fix.log
main.log

Notes for reviewers

[Copilot]

Pull request overview

This PR adjusts the x86bootdisk helper tool and build scripts to split BIOS vs UEFI boot image generation using Cargo features, with the goal of making x86_64 QEMU boot reliably by avoiding incompatible bootloader build paths.

Changes:

• Introduces bios/uefi feature flags in x86bootdisk and gates bootloader usage accordingly.
• Updates x86_64 Makefile image targets to build x86bootdisk with --no-default-features and the appropriate boot feature.
• Removes multilingual = false from the mdBook configuration.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
x86bootdisk/src/main.rs	Feature-gates BIOS/UEFI boot paths and adjusts CLI defaulting behavior based on enabled features.
x86bootdisk/Cargo.toml	Adds bios/uefi features and disables bootloader default features to control build paths.
mdbook/book.toml	Simplifies mdBook config by removing the explicit multilingual setting.
Makefile	Builds BIOS and UEFI images by selecting the corresponding x86bootdisk feature set.

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