stdlib/test-bz22786: Avoid spurious test failures using alias mappings

On systems without enough random-access memory, stdlib/test-bz22786
will go deeply into swap and time out, even with a substantial
TIMEOUTFACTOR.  This commit adds a facility to construct repeating
strings with alias mappings, so that the requirement for physical
memory, and uses it in stdlib/test-bz22786.

support/blob_repeat.h

@@ -0,0 +1,44 @@
+/* Repeating a memory blob, with alias mapping optimization.
+   Copyright (C) 2018 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
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SUPPORT_BLOB_REPEAT_H
+#define SUPPORT_BLOB_REPEAT_H
+
+#include <stdbool.h>
+#include <stddef.h>
+
+struct support_blob_repeat
+{
+  void *start;
+  size_t size;
+  bool use_malloc;
+};
+
+/* Return an allocation of COUNT elements, each of ELEMENT_SIZE bytes,
+   initialized with the bytes starting at ELEMENT.  The memory is
+   writable (and thus counts towards the commit charge).  In case of
+   on error, all members of the return struct are zero-initialized,
+   and errno is set accordingly.  */
+struct support_blob_repeat support_blob_repeat_allocate (const void *element,
+                                                         size_t element_size,
+                                                         size_t count);
+
+/* Deallocate the blob created by support_blob_repeat_allocate.  */
+void support_blob_repeat_free (struct support_blob_repeat *);
+
+#endif /* SUPPORT_BLOB_REPEAT_H */