nptl: Move pthread_cond_destroy implementation into libc

It is necessary to export __pthread_cond_destroy from libc because
the C11 condition variable needs it and is still left in libpthread.

This is part of the libpthread removal project:

  <https://sourceware.org/ml/libc-alpha/2019-10/msg00080.html

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