Kernel: Create a stub mcontext for aarch64

The struct only has a stub integer so that the size is the same for C
and C++. Something caught by CLion.

This commit was made with mrkct's help!

Kernel/Arch/aarch64/mcontext.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Platform.h>
+#include <Kernel/API/POSIX/sys/types.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct __attribute__((packed)) __mcontext {
+    int stub;
+};
+
+#ifdef __cplusplus
+}
+#endif