Build flag for an SP-optimized library.

This ffi is used in chain binaries as well as SP binaries.
The SP binaries do not need most of it, for example the SP binaries do not need:
- FVM runner
- WASM actors
- Genesis stuff
- wallet operations

A build flag to skip unnecessary pieces will lighten SP binaries, which helps Curio fit on smaller machines while having a large and growing tool bundle.

