Fix file corruption happening on Android 32 bit systems.

It is a bad idea to set CONFIG_FILEPOS_64 from outside.
Bcmastroska2 now follows standart _FILE_OFFSET_BITS to determine whether off_t is a 32 bit or 64 bit integer.
This mistake was creating invalid function pointer cast with the bctoolbox's VFS api (who follows the system off_t).