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/isa/riscv/abi.rs

@@ -0,0 +1,12 @@
+//! RISC-V ABI implementation.
+//!
+//! This module implements the RISC-V calling convention through the primary `legalize_signature()`
+//! entry point.
+
+use ir::Signature;
+use settings as shared_settings;
+
+/// Legalize `sig` for RISC-V.
+pub fn legalize_signature(_sig: &mut Signature, _flags: &shared_settings::Flags) {
+    // TODO: Actually do something.
+}