[$] Control-flow integrity in 5.13

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