Build flag for an SP-optimized library.

[snadrus]

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.