Config.uk: Fix dependencies with libunwind

Config.uk: Fix dependencies with libunwind

`libcxxabi` and `libunwind` create circular
dependencies. Since libraries like `libcompiler-rt`
and `cxx` depend on `libunwind`, it's more intuitive
to remove the `libcxxabi` requirement of `libunwind`
and make `libunwind` select `libcxxabi`.

Signed-off-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Approved-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #2