Parameterize op_t from memcopy.h

It moves the op_t definition out to an specific header, adds
the attribute 'may-alias', and cleanup its duplicated definitions.

Checked with a build and check with run-built-tests=no for all major
Linux ABIs.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

sysdeps/generic/string-optype.h

@@ -0,0 +1,24 @@
+/* Define a type to use for word access.  Generic version.
+   Copyright (C) 2023 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 _STRING_OPTYPE_H
+#define _STRING_OPTYPE_H 1
+
+typedef unsigned long int __attribute__ ((__may_alias__)) op_t;
+
+#endif /* string-optype.h */