malloc: Sync dynarray with gnulib

It syncs with gnulib version a8bac4d49.  The main changes are:

  - Remove the usage of anonymous union within DYNARRAY_STRUCT.
  - Use DYNARRAY_FREE instead of DYNARRAY_NAME (free) so that
    Gnulib does not change 'free' to 'rpl_free'.
  - Use __nonnull instead of __attribute__ ((nonnull ())).
  - Use __attribute_maybe_unused__ instead of
    __attribute__ ((unused, nonnull (1))).
  - Use of _Noreturn instead of _attribute__ ((noreturn)).

The only difference with gnulib is:

--- glibc
+++ gnulib
@@ -18,6 +18,7 @@

 #include <dynarray.h>
 #include <stdio.h>
+#include <stdlib.h>

 void
 __libc_dynarray_at_failure (size_t size, size_t index)
@@ -27,7 +28,6 @@
   __snprintf (buf, sizeof (buf), "Fatal glibc error: "
               "array index %zu not less than array length %zu\n",
               index, size);
- __libc_fatal (buf);
 #else
  abort ();
 #endif

It seems a wrong sync from gnulib (the code is used on loader and
thus it requires __libc_fatal instead of abort).

Checked on x86_64-linux-gnu.