cheats: fall back to content path for game-specific files

[Splaser]

Runtime-created cheats may fail to persist when no cheat file has been loaded beforehand.

Game-specific cheat filenames were derived from runloop_st->name.cheatfile only. When a cheat is created from the menu without first loading an existing .cht file, this value can be empty, causing cheat_manager_get_game_specific_filename() to return 0 and preventing the cheat list from being saved.

Fall back to the current content basename/path when no cheatfile name is available, while preserving the existing cheatfile-based behavior when it is set.

Tested on Android aarch64:

• launched content without loading an existing .cht file
• added a runtime cheat entry from Quick Menu > Cheats
• closed content and verified a game-specific .cht file was created
• relaunched the same content and verified the cheat list persisted