[$] Control-flow integrity in 5.13

Among the many changes merged for the 5.13 kernel is support for the LLVM
control-flow integrity (CFI) mechanism. CFI defends against exploits by
ensuring that indirect function calls have not been redirected by an
attacker. Quite a bit of work was needed to make this feature work well
for the kernel, but the result appears to be production-ready and able to
defend Linux systems from a range of attacks.