MDEV-9564 - added s390x to lib64 INSTALL_LIBDIR handling

Adjusted INSTALL_LIBDIR detection so that it is set to "lib64" on any 64bit
system (not only x86_64). New condition is insipired by GNUInstallDirs cmake
module.

cmake/install_layout.cmake

@@ -137,7 +137,7 @@ SET(INSTALL_SCRIPTDIR_RPM               "bin")
 SET(INSTALL_SYSCONFDIR_RPM		"/etc")
 SET(INSTALL_SYSCONF2DIR_RPM             "/etc/my.cnf.d")
 #
-IF(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64")
+IF(CMAKE_SIZEOF_VOID_P EQUAL 8)
   SET(INSTALL_LIBDIR_RPM                "lib64")
   SET(INSTALL_PLUGINDIR_RPM             "lib64/mysql/plugin")
 ELSE()