stddef: Introduce __TRAILING_OVERLAP()

Introduce underlying __TRAILING_OVERLAP() macro to let callers apply
atributes to trailing overlapping members.

For instance, the code below:

| struct flex {
| 	size_t count;
| 	int data[];
| };

| struct {
| 	struct flex f;
| 	struct foo a;
| 	struct boo b;
| } __packed instance;

can now be changed to the following, and preserve the __packed
attribute:

| __TRAILING_OVERLAP(struct flex, f, data, __packed,
| 	struct foo a;
| 	struct boo b;
| ) instance;

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/f80c529b239ce11f0a51f714fe00ddf839e05f5e.1758115257.git.gustavoars@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>