[Bug] Realign and size check EECONFIG structures (#20541)

Co-authored-by: Nick Brassel <[email protected]>

quantum/keycode_config.h

@@ -16,6 +16,10 @@
 
 #pragma once
 
+#ifdef __cplusplus
+#    define _Static_assert static_assert
+#endif
+
 #include "eeconfig.h"
 #include "keycode.h"
 #include "action_code.h"
@@ -43,4 +47,6 @@ typedef union {
     };
 } keymap_config_t;
 
+_Static_assert(sizeof(keymap_config_t) == sizeof(uint16_t), "Keycode (magic) EECONFIG out of spec.");
+
 extern keymap_config_t keymap_config;