s390x: notice regarding incompatbile lmdb format

[bolives-hax]

Hey I build monerod and monero-wallet-cli for s390x and ran it on one of my hosts (testnet fauccet worked) but while I now know that copying over the lmdb file from a little endian host such as my x86 host to my big endian s390x host isn't possible due to the endianess mismatch. It would be great to maybe make monerod be aware of the differences between the 2 formats and report it before exiting with an error as other people may not come across this issue or draw the conlcusion themself as to why that happened.

Other than that im happy to report monero-wallet-cli + monerod (--testnet) seem to work fine on s390x against all initial expectations and i was able to send funds in a circle on testnet from and to my x86 host and back to s390x