Cargo `exclude_entrypoint` feature doesn't appear to be used correctly

[0xekez]

The Solana docs seem to suggest that an exclude-entrypoint feature should be provided which stops the entryponit! macro invocation from being run.

The Rust example program seems to have a different no-entrypoint feature that is unused. I figure that should probably be renamed to exclude-entrypoint and the entrypoint macro invocation should look something like this:

#[cfg(not(feature = "exclude_entrypoint"))]
entrypoint!(process_instruction);

I'm happy to put up a PR for this if what I'm saying sounds reasonable.