MB-12802: Set platform to "unknown"

Set the platform to "unknown" if we fail to determine the
platform.

Change-Id: I604bd881afef4d3016d36b5454f90dc9e9fa77c5
Reviewed-on: http://review.couchbase.org/43767
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>