[PATCH] x86: Add __sigsetjmp_cancel and __setjmp_cancel

[Florian Weimer]

* H. J. Lu:

> +/* Function used in the macros.  */
> +struct __jmp_buf_tag;
> +extern int __sigsetjmp_cancel (struct __jmp_buf_tag *__env, int __savemask)
> +  __THROWNL __attribute_returns_twice__;

How is __THROWNL correct here?  We do unwind through this function.
The __EXCEPTIONS guard is not effective because of LTO, and some
applications undefine __EXCEPTIONS to avoid the table-based
cancellation handlers.