host_memory: merge adjacent placeholder mappings on Linux

Track the private anonymous placeholder mappings created by Unmap() and
wherever possible, replace existing placeholders with larger ones
instead of creating many small ones.

This helps with the buildup of mappings in /proc/YUZU_PID/maps after a
longer gaming session, improving stability without having to increase
vm.max_map_count to a ridiculous value. The amount of placeholder
mappings will no longer outgrow the amount of actual memfd mappings in
cases of high memory fragmentation.