ppc64_linux26: build binaries as 64-bit by default

lwp doesnt work since ucontext/setjmp are incomplete when running 32-bit
binaries with the 64-bit kernel.

Reviewed-on: http://gerrit.openafs.org/5365
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 39b788eeafe27d21ae6b4fd3cfeb59ce5dc70077)

Change-Id: Ic4fd5c1a861db8e96dd58a79d7fb2ea73ab4f0d6
Reviewed-on: http://gerrit.openafs.org/9605
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>