Set _FILE_OFFSET_BITS=64 again when necessary

This essentially reverts db553c81. It
turns out that (for better or worse) quite a few places access files
without going through KFile, so it is worth making sure they can deal
with files larger than 2Gb on 32-bit systems.

Reviewed by: Harald Sitter <sitter@kde.org>

CCBUG: 165449