Add a legalize_signature method to TargetIsa.

This entry point will be used for controlling ABI conventions when
legalizing.

Provide an empty implementation for RISC-V and let the other ISAs crash
in legalization.

This is just the scaffolding. We still need to:

- Rewrite the entry block arguments to match the legalized signature.
- Rewrite call and return instructions.
- Implement the legalize_signature() function for all ISAs.
- Add shared generic types to help with the legalize_signature()
  functions.

lib/cretonne/src/ir/function.rs

@@ -64,11 +64,6 @@ impl Function {
     pub fn new() -> Function {
         Self::with_name_signature(FunctionName::default(), Signature::new())
     }
-
-    /// Get the signature of this function.
-    pub fn own_signature(&self) -> &Signature {
-        &self.signature
-    }
 }
 
 impl Display for Function {