Build s_nan* objects from a generic template

This requires adding a macro to synthesize the call
to __strto*_nan.  Since this is likely to be the only
usage ever for strto* functions in generated libm
calls, a dedicated macro is defined for it.

math/s_nan_template.c

@@ -0,0 +1,37 @@
+/* Return quiet nan.
+   Copyright (C) 1997-2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.
+
+   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/>.  */
+
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ieee754.h>
+
+
+FLOAT
+M_DECL_FUNC (__nan) (const char *tagp)
+{
+  return M_STRTO_NAN (tagp, NULL, 0);
+}
+
+declare_mgen_alias (__nan, nan)
+
+#if M_LIBM_NEED_COMPAT (nan)
+declare_mgen_libm_compat (__nan, nan)
+#endif