gcc-plugins: Reorganize gimple includes for GCC 13

The gimple-iterator.h header must be included before gimple-fold.h
starting with GCC 13. Reorganize gimple headers to work for all GCC
versions.

Reported-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/all/20230113173033.4380-1-palmer@rivosinc.com/
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>