Migrate code from cargo-xbuild to -Zbuild-std

[phil-opp]

This pull request implements building using cargo's new build-std instead of using the cargo-xbuild crate.

To enable the build-std functionality, we set the unstable.build-std configuration key in the .cargo/config.toml file. This flag was introduced very recently in rust-lang/cargo#8393, so it only works on the latest nightlies. Unfortunately, rustfmt doesn't build on these nightlies yet, so you have to use rustup update nightly --force to install them (results in skipping the rustfmt component).

Functionality-wise, the build-std feature behaves almost identical to cargo-xbuild. The only difference is that it doesn't enable the mem feature of the compiler_builtins crate, which defines the memcpy, memset, etc operations. This is a known issue, which is hopefully fixed soon. Until then, we can work around it by adding a dependency on the rlibc crate. To ensure that this crate is linked even though it is not directly used, we need to add an extern crate rlibc statement to our main.rs.

After these changes, our kernel is buildable through a normal cargo build. The cargo run and cargo test commands will also work as expected, provided the latest bootimage version is used.

This PR additionally rewrites our CI script based on the GitHub Actions provided by the actions-rs organization. This makes Rust's error messages compatible with GitHubs annotations, so that any build errors and warnings are shown inline at the correct line.

The required changes to the blog post will be done in a separate follow-up PR.

[phil-opp]

The additional changes required to the other post-x branches after merging this PR were minimal, so I pushed them directly.

[phil-opp]

I submitted #836 to update the blog posts for these changes.