use new btxld when building world in older environment

This is a direct commit to the branch.
This is for the benefit of those who define LOADER_ZFS_SUPPORT but
should't hurt anybody else.
Build of zfsboot requires new feature of btxld that it obtained
(via MFC) in r192697: "merge 177933. Allow for a zero length 'loader'"
(btxld is a build tool and a cross-build tool)
Please note that __FreeBSD_version was bumped some time after r192697
for an unrelated change.

Reviewed by: des
Approved by: jhb (mentor)