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/codegen/src/isa/x64/abi.rs

@@ -708,6 +708,7 @@ impl ABIMachineSpec for X64ABIMachineSpec {
         regs: &[Writable<RealReg>],
     ) -> Vec<Writable<RealReg>> {
         let mut regs: Vec<Writable<RealReg>> = match call_conv {
+            CallConv::Tail => unimplemented!(),
             CallConv::Fast | CallConv::Cold | CallConv::SystemV | CallConv::WasmtimeSystemV => regs
                 .iter()
                 .cloned()
@@ -823,6 +824,7 @@ fn get_intreg_for_retval(
     retval_idx: usize,
 ) -> Option<Reg> {
     match call_conv {
+        CallConv::Tail => unimplemented!(),
         CallConv::Fast | CallConv::Cold | CallConv::SystemV => match intreg_idx {
             0 => Some(regs::rax()),
             1 => Some(regs::rdx()),
@@ -851,6 +853,7 @@ fn get_fltreg_for_retval(
     retval_idx: usize,
 ) -> Option<Reg> {
     match call_conv {
+        CallConv::Tail => unimplemented!(),
         CallConv::Fast | CallConv::Cold | CallConv::SystemV => match fltreg_idx {
             0 => Some(regs::xmm0()),
             1 => Some(regs::xmm1()),