Increase size limit of baloo index for 64-bit machines

CHANGELOG: On 64-bit systems baloo allows now > 5 GB index storage.

Increase size limit of baloo index for 64-bit machines to avoid crashs after > 5GB of index size.
(Better would be additional out-of-space handling, but ATM baloo has zero checks for that)

The size limit for 32-bit is still 1GB, like before (there was a silent overflow from 5GB to 1GB in the computation), people with large homes will still get random segfaults on 32-bit.

Patch based on patch from Hao Zhang, Bug 364475

REVIEW: 128885
BUG: 364475