y2038: Convert cnd_timedwait to support 64 bit time

y2038: Convert cnd_timedwait to support 64 bit time

The cnd_timedwait function has been converted to support 64 bit time.
It was also necessary to provide Linux specific copy of it to avoid
problems on i686-gnu (i.e. HURD) port, which is not providing
pthread_cond_timedwait() supporting 64 bit time.

Moreover, a linux specific copy of thrd_priv.h header file has been
added as well.

The cnd_timedwait is a wrapper on POSIX threads to provide C11 standard
threads interface. It directly calls __pthread_cond_timedwait64().

Build tests:
./src/scripts/build-many-glibcs.py glibcs

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>