Fix sret for AArch64 (#4634)

* Fix sret for AArch64

AArch64 requires the struct return address argument to be stored in the x8
register. This register is never used for regular arguments.

* Add extra sret tests for x86_64

cranelift/codegen/src/isa/aarch64/abi.rs

@@ -143,6 +143,7 @@ impl ABIMachineSpec for AArch64MachineDeps {
             let (rcs, reg_types) = Inst::rc_for_type(param.value_type)?;
 
             if let ir::ArgumentPurpose::StructArgument(size) = param.purpose {
+                assert_eq!(args_or_rets, ArgsOrRets::Args);
                 let offset = next_stack as i64;
                 let size = size as u64;
                 assert!(size % 8 == 0, "StructArgument size is not properly aligned");
@@ -156,6 +157,24 @@ impl ABIMachineSpec for AArch64MachineDeps {
                 continue;
             }
 
+            if let ir::ArgumentPurpose::StructReturn = param.purpose {
+                // FIXME add assert_eq!(args_or_rets, ArgsOrRets::Args); once
+                // ensure_struct_return_ptr_is_returned is gone.
+                assert!(
+                    param.value_type == types::I64,
+                    "StructReturn must be a pointer sized integer"
+                );
+                ret.push(ABIArg::Slots {
+                    slots: smallvec![ABIArgSlot::Reg {
+                        reg: xreg(8).to_real_reg().unwrap(),
+                        ty: types::I64,
+                        extension: param.extension,
+                    },],
+                    purpose: ir::ArgumentPurpose::StructReturn,
+                });
+                continue;
+            }
+
             // Handle multi register params
             //
             // See AArch64 ABI (https://github.com/ARM-software/abi-aa/blob/2021Q1/aapcs64/aapcs64.rst#642parameter-passing-rules), (Section 6.4.2 Stage C).