introduce vm.anon_low_pref option to optimize cluster anon behavior.
Enabling this option, will stabilize map->anon_loc for searching clustering candidates allows better reuse of returned memory regions.
This option is disabled by default.
Continuously updating map->anon_loc could impact jemalloc metadata consumption when handling virtual address upward shifts.
By stabilizing the address of map->anon_loc, we can mitigate metadata consumption in jemalloc.