MDEV-34348: Consolidate cmp function declarations

Partial commit of the greater MDEV-34348 scope.
MDEV-34348: MariaDB is violating clang-16 -Wcast-function-type-strict

The functions queue_compare, qsort2_cmp, and qsort_cmp2
all had similar interfaces, and were used interchangable
and unsafely cast to one another.

This patch consolidates the functions all into the
qsort_cmp2 interface.

Reviewed By:
============
Marko Mäkelä <marko.makela@mariadb.com>

include/my_cmp.h

@@ -0,0 +1,25 @@
+/* Copyright (c) 2024, MariaDB Corporation.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA */
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+typedef int (*qsort_cmp)(const void *, const void *);
+typedef int (*qsort_cmp2)(void *param, const void *a, const void *b);
+#ifdef __cplusplus
+}
+#endif