C11 threads: Move implementation to sysdeps/pthread

so it gets shared by nptl and htl. Also add htl versions of thrd_current and
thrd_yield.

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

sysdeps/pthread/call_once.c

@@ -0,0 +1,31 @@
+/* C11 threads call once implementation.
+   Copyright (C) 2018-2020 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 <stdalign.h>
+
+#include "thrd_priv.h"
+
+void
+call_once (once_flag *flag, void (*func)(void))
+{
+  _Static_assert (sizeof (once_flag) == sizeof (pthread_once_t),
+		  "sizeof (once_flag) != sizeof (pthread_once_t)");
+  _Static_assert (alignof (once_flag) == alignof (pthread_once_t),
+		  "alignof (once_flag) != alignof (pthread_once_t)");
+  __pthread_once ((pthread_once_t *) flag, func);
+}