KCFI support in GCC

[kees]

GCC still needs support for KCFI, most especially for x86_64 and arm64.

Details on the Clang implementation were discussed in the 2022 Linux Plumbers Conference. PDF and video link at the end:
https://lpc.events/event/16/contributions/1315/

Prior draft (though without the needed specific binary output requirements) for arm64:
https://lore.kernel.org/lkml/20230325085416.95191-1-ashimida.1990@gmail.com/

[kees]

https://lore.kernel.org/linux-hardening/20250821064202.work.893-kees@kernel.org/