gcc-plugins: Remove TODO_verify_il for GCC >= 16

gcc-plugins: Remove TODO_verify_il for GCC >= 16

GCC now runs TODO_verify_il automatically[1], so it is no longer exposed to
plugins. Only use the flag on GCC < 16.

Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=9739ae9384dd7cd3bb1c7683d6b80b7a9116eaf8
Suggested-by: Christopher Fore <csfore@posteo.net>
Link: https://lore.kernel.org/r/20250920234519.work.915-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>