Disable vectored positional IO if no 64bit support

Disable vectored positional IO if no 64bit support

In order to support vectored positional I/O, and large files, we
need there to be an implementation of preadv64 and pwritev64. If
this isn't present, then just disable HAVE_PIOV.

(cherry picked from commit 98a0c2f47b8641e3e31ed7d2f7d84c2eb484ef51)
Reviewed-on: http://gerrit.openafs.org/4153
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Change-Id: I4e199c748d7b88ff7076106b19c3dec0e60be39a
Reviewed-on: http://gerrit.openafs.org/4160
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>