Merge #378

378: Add xtask crate for kernel building r=stlankes a=mkroening

Closes https://github.com/hermitcore/rusty-hermit/issues/188.

This adds an xtask package, with two subcommands `build` and `clippy`.
`build` builds the kernel with the correct arguments and does any necessary post processing (symbol renaming).
`clippy` runs clippy on the kernel for multiple architectures (though only `x86_64` at this time - see https://github.com/hermitcore/libhermit-rs/issues/381

).

Co-authored-by: Martin Kröning <mkroening@posteo.net>