nptl: Move core condition variable functions into libc

Onl pthread_cond_clockwait did not have a forwarder, so it needs
a new symbol version.

Some complications arise due to the need to supply hidden aliases,
GLIBC_PRIVATE exports (for the C11 condition variable implementation
that still remains in libpthread) and 64-bit time_t stubs.

pthread_cond_broadcast, pthread_cond_signal,  pthread_cond_timedwait,
pthread_cond_wait, pthread_cond_clockwait have been moved using
scripts/move-symbol-to-libc.py.

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