[$] Controlling shadow-stack allocation in clone3()

User-space shadow stacks are a relatively new feature in Linux; support wasonly added for 6.6, and is limited to the x86architecture. As support for other architectures (including arm64 and RISC-V) approaches readiness,though, more thought is going into the API for this feature. As a recentdiscussion on the integration of shadow stacks with the clone3() system call shows, there arestill some details to be worked out.