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

[Nathan Chancellor via llvm-commits]

nathanchance wrote:

Sorry for the late testing but the final version (tested at 83f751a001fa57dd8eaa60f08ed420e90170e11d) does not appear to survive `allmodconfig` (which does not have `CONFIG_THUMB2_KERNEL=y`) for me. For example, with Linus's latest tree:

```
$ make -skj"$(nproc)" ARCH=arm LLVM=1 clean allmodconfig drivers/net/wireless/realtek/rtlwifi/pci.o
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
error: out of range pc-relative fixup value
18 errors generated.
make[9]: *** [scripts/Makefile.build:287: drivers/net/wireless/realtek/rtlwifi/pci.o] Error 1
...
```

but this happens in many many files within in this build.

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