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>

sysdeps/unix/sysv/linux/thrd_priv.h

@@ -0,0 +1,27 @@
+/* Internal C11 threads definitions - linux version
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sysdeps/pthread/thrd_priv.h>
+
+#if __TIMESIZE == 64
+# define __cnd_timedwait64 __cnd_timedwait
+#else
+extern int __cnd_timedwait64 (cnd_t *restrict cond, mtx_t *restrict mutex,
+                              const struct __timespec64 *restrict time_point);
+libpthread_hidden_proto (__cnd_timedwait64)
+#endif