Replace ASM files with #[naked] functions

[iankronquist]

Let's go pure rust on this. We're already using inline assembly, let's go all the way.

Remember to add a feature flag to lib.rs.

rust-lang/rust#32410

[s-mcf]

We can use the x86-interrupt ABI and avoid writing inline assembly entirely.

Tracking issue: rust-lang/rust#20180
PR with some meager documentation: rust-lang/rust#39832
No RFC exists AFAIK.

[iankronquist]

Deprecated in favor of a UEFI runtime service.