main: add a debug symbol for service level controller
main: add a debug symbol for service level controller

It's notoriously hard to find the service level controller symbol
(possible by guessing the offset based on system_distributed_keyspace
address, but it's very cumbersome). To make the debugging process
easier, the symbol is exported via the `debug` namespace.

Closes #8506

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla