feat: add bare metal support

[Godones]

I want to use this library on an OS implemented in Rust, which is not Linux/macOS/Windows, and the library lacks support for bare metal OS. Since we cannot know how these bare metal OS modify the code segment, I simply overwrite the memory in the ArchCodeManipulator::write_code function. Bare metal OS developers should complete the relevant operations before and after updating the key.

[Evian-Zhang]

I'm very glad to merge this PR. However, I know little about bare metal, and before merging this PR, I want to make sure things will go well. My question is about the section naming. Will the bare metal linker generate the correct __start/__stop symbols to mark the start and end of customized section? Is there any way to test in bare-metal environment for GitHub Actions CI? (If not, that's OK still. I will just mention it in the README)

[Godones]

Typically, OS or other programs developed on bare metal include a custom linker script, so I think we should add some documentation here to tell people who use this library on bare metal that they should manually add these symbols to their linker script.

[Godones]

I can't find a better way to test in bare-metal environment for GitHub Actions CI because this may require a simple OS running on bare metal.

[Evian-Zhang]

OK, thank you for your contributions! :)