[clang] [llvm] [ARM][KCFI] Add backend support for Kernel Control-Flow Integrity (PR #163698)

[Kees Cook via cfe-commits]

kees wrote:

> > With `CONFIG_CFI=y` and `CONFIG_THUMB2_KERNEL=y`, I see many
> > ```
> > error: out of range pc-relative fixup value
> > ```
> >   
> > with the latest revision. Normal ARM mode works fine still from what I can tell.
> 
> I forgot we could even compile Linux with Thumb2! I did all my testing of Thumb2 under userspace. Thanks for finding that; I will take a look.

Okay, this should be fixed now, and a new regression test added. I've validated a `CONFIG_THUMB2_KERNEL=y` Linux build, boot, and LKDTM test run.

https://github.com/llvm/llvm-project/pull/163698