[Bug target/81842] -fcf-protection -mcet is incompatible with makecontext family functions

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81842

--- Comment #12 from H.J. Lu <hjl.tools at gmail dot com> ---
Maybe we should just simply poison <ucontext.h> to issue an error
if SHSTK is enabled.