[$] User-space shadow stacks (maybe) for 6.4

Support for shadow stacks on the x86 architecture has been long in coming;LWN first covered this work in 2018. Afterfive years and numerous versions, though, it would appear thatuser-space shadow stacks on x86 might just be supported in the 6.4 kernelrelease. Getting there has required a few changes since we last caught up with this work in early 2022.