support: Implement <support/xthread.h> key create/delete

Expose xpthread_key_create and xpthread_key_delete wrappers
for tests.

support/xpthread_key_create.c

@@ -0,0 +1,28 @@
+/* pthread_key_create with error checking.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   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 <support/xthread.h>
+
+pthread_key_t
+xpthread_key_create (void (*destr_function) (void *))
+{
+  pthread_key_t key;
+
+  xpthread_check_return ("pthread_key_create",
+                         pthread_key_create (&key, destr_function));
+  return key;
+}