pthread_cancel async-signal-safe

[Florian Weimer]

* Adhemerval Zanella via Libc-alpha:

> Florian, I recall that you had investigate some issue regarding C++
> exceptions and signal handler that you presented on previous GNU 
> Cauldron, so I wonder if you might have an idea how we can fix it.

I didn't realize that pthread_cancel was required to be
async-signal-safe.  We don't have an async-signal-safe dlopen and we
can't block signals during dlopen because that breaks the web browser
sandboxes.

Are sure that pthread_cancel is on the list?  I don't see it in
signal-safety(7).