[i386] PR81842 [CET] -fcf-protection -mcet is incompatible with makecontext family functions

[Tsimbalist, Igor V]

-fcf-protection -mcet is incompatible with makecontext family functions
since they can't properly set up and destroy shadow stack pointer. This
change provides a mechanism to help detection shadow stack compatibility.
The current proposal is to add -mcheck-shstk-compat option which will
predefine __CHECK_SHSTK_COMPAT__ macro. The option will be
set on by default.  Then we can add a code

#if defined __SHSTK__ && defined __CHECK_SHSTK_COMPAT__
# error This source is incompatible with -mshstk
#endif

to <ucontext.h>.

Ok for trunk?

Igor
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