pthread_cancel async-signal-safe

[Florian Weimer]

* Adhemerval Zanella:

> The pthread_cancel interoperability with C++ seems a heavy burden, but I'm
> not sure which better options we do have here.

It's not just C++, it's needed for -fexception support in C and other
languages.

Furthermore, the out-of-line DWARF unwinding tables can be made
considerably safer than on-stack pointers used by the longjmp-based
mechanism.

Thanks,
Florian
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