bootstrapping: add support for big-endian systems

[cryptozoidberg]

1. do not download bootstrap blockchain if the system is big-endian (as a first aid, additional research required);
2. make additional predownload file for big-endian or, better, implement ad-hoc conversion;
3. fix the issue with 64-bit ARM systems being detected as 32-bit, the issue likely is in contrib/epee/include/os_defenitions.h;
4. consider fixing 32-bit LMDB issue ((-30792)MDB_MAP_FULL: Environment mapsize limit reached, [message]: Unable to mdb_put)

Originally submitted by sowle
Moved from hyle-team/zano#322