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

[Nathan Chancellor via cfe-commits]

nathanchance 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.

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