kernel: add an architecture specific structs header

Add the ability to define architecture specific structures, notably
the ability to extend struct _cpu with per-CPU arch-specific stuff that
can be accessed with _current_cpu->arch.* similarly to _current->arch.*
for per-thead architecture data.

This is opt-in for architectures that want to benefit from this,
otherwise empty defaults are provided. A placeholder for ARM64 is
included to show the pattern.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

include/arch/arm64/structs.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) BayLibre SAS
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_INCLUDE_ARM64_STRUCTS_H_
+#define ZEPHYR_INCLUDE_ARM64_STRUCTS_H_
+
+/* Per CPU architecture specifics */
+struct _cpu_arch {
+	/* content coming soon */
+};
+
+#endif /* ZEPHYR_INCLUDE_ARM64_STRUCTS_H_ */