Add option for forcing a 64bit hash on 32bit systems

[Érico Nogueira]

Hi!

As seen in [1], aspell has a rather unfortunate design limitation in its
compiled dictionary format. This not only makes it impossible to share
these files between systems with different endianness, but, if compiled
with default settings, makes sharing impossible between 32bit and 64bit
systems as well.

The only compatibility option offered is forcing 32bit hashes for all
systems, which makes 64bit systems incapable of reading 64bit
dictionaries. Since most systems nowadays (especially ones where aspell
would be used) are 64bit, it feels kind of backwards to force 32bit
dicts. Would it be possible, then, to force 64bit dicts on 32bit
systems? Would a contribution that adds this possibility be accepted?

[1] 
http://aspell.net/man-html/Using-32_002dBit-Dictionaries-on-a-64_002dBit-System.html

Thanks,
Érico