Enabling using Spike as a library

[mslijepc]

This PR should introduce API to enable some external simulator connect to Spike.

Steps:

1. External simulator (e.g. wrapper of a simulator, or a bridge) has be derived from abstract_sim_if_t class and has to implement abstract functions do_load and do_store.
2. cfg_t is now extended with std::optional<abstract_sim_if_t*> external_simulator field. This field should be filled with pointer to simulator object from Step 1.
3. external_sim_device_t should be constructed with pointer from step 2, and create bus_t with fallback parameter of external_sim_device_t.

For all addresses which are not in Spike address space, bus will issue load/store requests to fallback branch, i.e. external simulator