cranelift: introduce a new WasmtimeAppleAarch64 calling convention

The previous choice to use the WasmtimeSystemV calling convention for
apple-aarch64 devices was incorrect: padding of arguments was
incorrectly computed. So we have to use some flavor of the apple-aarch64
ABI there.

Since we want to support the wasmtime custom convention for multiple
returns on apple-aarch64 too, a new custom Wasmtime calling convention
was introduced to support this.

cranelift/codegen/src/machinst/abi_impl.rs

@@ -652,7 +652,8 @@ impl<M: ABIMachineSpec> ABICalleeImpl<M> {
                 || call_conv.extends_baldrdash()
                 || call_conv.extends_windows_fastcall()
                 || call_conv == isa::CallConv::AppleAarch64
-                || call_conv == isa::CallConv::WasmtimeSystemV,
+                || call_conv == isa::CallConv::WasmtimeSystemV
+                || call_conv == isa::CallConv::WasmtimeAppleAarch64,
             "Unsupported calling convention: {:?}",
             call_conv
         );