Use the ppc64le_linuxXX sysname for ppc64le builds

Use the ppc64le_linuxXX sysname for ppc64le builds

Commit 191e18eb (Open ppc64le_linux sysname space) added the
ppc64le_linux26 sysname, but it still must be manually specified when
running on ppc64le. Use the ppc64le_linux26 by default on ppc64le, so
we can compile without needing to specify an explicit sysname.

Reviewed-on: https://gerrit.openafs.org/13593
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 4b6a4ff31a4197504bbcf2d4c14c24dee672d40e)

Change-Id: Icf8f8b42c499dc42bf5d637dae5ad3e261e68512
Reviewed-on: https://gerrit.openafs.org/13637
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>