Config.uk: Fix dependencies with libcxxabi

Config.uk: Fix dependencies with libcxxabi

`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 <razvand@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #3