Add xtask crate for kernel building

.cargo/config.toml

.github/workflows/aarch64.yml

.github/workflows/clippy.yml

.github/workflows/doc.yml

.github/workflows/publish_docs.yml

.github/workflows/x86.yml

.gitlab-ci.yml

Cargo.toml

Dockerfile

xtask/.gitignore

xtask/Cargo.toml

xtask/src/flags.rs

xtask/src/main.rs

xtask/src/rustc.rs