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

[Kees Cook via cfe-commits]

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+; RUN: llc -mtriple=thumbv6m-none-eabi < %s | FileCheck %s
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kees wrote:

Okay, I've got this done now. It required moving the MIR/ISEl/KCFI checks up out of the way, but I think it's all still readable. I dropped some comments and collected others to after the update_llc_test_checks-touching areas.

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