Reinstate generic features-time64.h

The a4ed0471d7 removed the generic version which is included by
features.h and used by Hurd.

Checked by building i686-gnu and x86_64-gnu with build-many-glibc.py.

sysdeps/generic/features-time64.h

@@ -0,0 +1,19 @@
+/* Features part to handle 64-bit time_t support.  Generic version.
+   Copyright (C) 2021-2024 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/>.  */
+
+/* The generic configuration only support _TIME_BITS=32.  */