Added FS functions to Archive and Archive_RomFS

Added pack_u32 and unpack_u64 helper functions to math_util.h
Not sure this is even worth defining as a new function, since it's such a simple operation (and it looks like it's just being used once). If you do need this, I would say it belongs somewhere in Common, because it's a general purpose routine and not actually specific to core emulation (e.g. it could be used elsewhere).