Implement legalize_signature for RISC-V.

Add an abi module with code that is probably useful to all ISAs when
implementing this function.

Add a unit() method to RegClassData which can be used to index the
register units in a class.

lib/cretonne/src/lib.rs

@@ -21,6 +21,7 @@ pub mod settings;
 pub mod sparse_map;
 pub mod verifier;
 
+mod abi;
 mod constant_hash;
 mod context;
 mod legalizer;