We were previously missing `Kernel/Arch`, which gets included by
`Kernel/API`.
While at it, remove `AK` from the list of copied headers for the
toolchain, as nothing in our LibC headers should ever publicly depend on
AK.
This is needed to avoid including LibC headers in Lagom builds.
Unfortunately, we cannot rely on the build machine to provide a
fully POSIX-compatible ELF header for Lagom builds, so we have to
use our own.
