libuafs: enable 64 bit mode on linux

libuafs: enable 64 bit mode on linux

already safe and being used on other platforms; might as well
here too.

Reviewed-on: http://gerrit.openafs.org/8139
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 71a5f2965d6a6ff3113e7d2f9cab7d687b502793)

Change-Id: I61673c179617377020aa95aa8d8d659f240f04e7
Reviewed-on: http://gerrit.openafs.org/8238
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>