nptl: Move pthread_exit into libc

The pthread_exit symbol was moved using
scripts/move-symbol-to-libc.py.  No new symbol version is needed
because there was a forwarder.

The new tests nptl/tst-pthread_exit-nothreads and
nptl/tst-pthread_exit-nothreads-static exercise the scenario
that pthread_exit is called without libpthread having been linked in.
This is not possible for the generic code, so these tests do not
live in sysdeps/pthread for now.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>