stddef: Introduce TRAILING_OVERLAP() helper macro

Add new TRAILING_OVERLAP() helper macro to create a union between
a flexible-array member (FAM) and a set of members that would
otherwise follow it. This overlays the trailing members onto the
FAM while preserving the original memory layout.

Co-developed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/aFG8gEwKXAWWIvX0@kspp


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