gcc-plugins: Always define CONST_CAST_GIMPLE and CONST_CAST_TREE

gcc-plugins: Always define CONST_CAST_GIMPLE and CONST_CAST_TREE

For gcc-16, the CONST_CAST macro family was removed. Add back what
we were using in gcc-common.h, as they are simple wrappers.

See GCC commits:
  c3d96ff9e916c02584aa081f03ab999292efbb50
  458c7926d48959abcb2c1adaa22458e27459a551

Suggested-by: Ingo Saitz <ingo@hannover.ccc.de>
Link: https://lore.kernel.org/lkml/ab6OKoay0OWkywjK@spatz.zoo
Fixes: 6b90bd4ba40b ("GCC plugin infrastructure")
Tested-by: Ivan Bulatovic <combuster@archlinux.us>
Tested-by: Christopher Cradock <christopher@cradock.myzen.co.uk>
Signed-off-by: Kees Cook <kees@kernel.org>