Select lib-compiler-rt as Kconfig dependency

Select lib-compiler-rt as Kconfig dependency

Commit 70bac65 (Split off functionality that depends on libunwind)
in lib-compiler-rt removed the circular dependency between it and
libunwind.
This change thus removes the workaround check for lib-compiler-rt and
more cleanly expresses it through Kconfig `select`.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #10