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.

Reviewed-on: http://gerrit.openafs.org/10151
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 1c0260c71185a41f1ec3d1c08f20c53c76f31cdd)

Change-Id: I4d58dc00bb42929d0695eb84fd00814d6e8bc30e
Reviewed-on: http://gerrit.openafs.org/10161
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>