Support userspace CET

CET is an x86 feature protecting against ROP/JOP by limiting possible indirect branch targets, and RETurns using shadow stacks. This improves the security especially for memory-unsafe userspace software, but is also useful for Rust, as there's usually a nontrivial amount of unsafe code somewhere. Some (many?) Linux distros including Gentoo, use CET in userspace by default.

Kernel space CET is most likely not desired at this point, due to the small size of the kernel.