cpp: Update structs to be compatible with C++

Both the `_cpu_arch` and `k_thread_runtime_stats` structs can have a
size of 0 in C, but will fail when building with C++. Add an extra
byte in those cases.

Signed-off-by: Yuval Peress <[email protected]>

Author: yperess

include/zephyr/arch/structs.h

@@ -31,6 +31,12 @@
 
 /* Per CPU architecture specifics (empty) */
 struct _cpu_arch {
+#ifdef __cplusplus
+	/* This struct will have a size 0 in C which is not allowed in C++ (it'll have a size 1). To
+	 * prevent this, we add a 1 byte dummy variable.
+	 */
+	uint8_t dummy;
+#endif
 };
 
 #endif

include/zephyr/kernel/thread.h

@@ -208,6 +208,15 @@ typedef struct k_thread_runtime_stats {
 
 	uint64_t idle_cycles;
 #endif
+
+#if __cplusplus && !defined(CONFIG_SCHED_THREAD_USAGE) &&                                          \
+	!defined(CONFIG_SCHED_THREAD_USAGE_ANALYSIS) && !defined(CONFIG_SCHED_THREAD_USAGE_ALL)
+	/* If none of the above Kconfig values are defined, this struct will have a size 0 in C
+	 * which is not allowed in C++ (it'll have a size 1). To prevent this, we add a 1 byte dummy
+	 * variable when the struct would otherwise be empty.
+	 */
+	uint8_t dummy;
+#endif
 }  k_thread_runtime_stats_t;
 
 struct z_poller {