Cranelift: Introduce the return_call and return_call_indirect instructions (#5679)

* Cranelift: Introduce the `tail` calling convention

This is an unstable-ABI calling convention that we will eventually use to
support Wasm tail calls.

Co-Authored-By: Jamey Sharp <jsharp@fastly.com>

* Cranelift: Introduce the `return_call` and `return_call_indirect` instructions

These will be used to implement tail calls for Wasm and any other language
targeting CLIF. The `return_call_indirect` instruction differs from the Wasm
instruction of the same name by taking a native address callee rather than a
Wasm function index.

Co-Authored-By: Jamey Sharp <jsharp@fastly.com>

* Cranelift: Implement verification rules for `return_call[_indirect]`

They must:

* have the same return types between the caller and callee,
* have the same calling convention between caller and callee,
* and that calling convention must support tail calls.

Co-Authored-By: Jamey Sharp <jsharp@fastly.com>

* cargo fmt

---------

Co-authored-by: Jamey Sharp <jsharp@fastly.com>

cranelift/interpreter/src/step.rs

@@ -391,6 +391,8 @@ where
             }
         }
         Opcode::CallIndirect => unimplemented!("CallIndirect"),
+        Opcode::ReturnCall => unimplemented!("ReturnCall"),
+        Opcode::ReturnCallIndirect => unimplemented!("ReturnCallIndirect"),
         Opcode::FuncAddr => unimplemented!("FuncAddr"),
         Opcode::Load
         | Opcode::Uload8