Windows: Cap Cache Size on X86

Since we know the cache size cannot be arbitrary size because it
must fit into contiguous process memory and because it is difficult
to compute the actual size limit, cap the size to 716800KB.

Change-Id: I850f489dc8a392875582e52d8d511dedf69aec9c
Reviewed-on: http://gerrit.openafs.org/10151
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>