Add sparc_linux26 sysname and params

A copy of sparc_linux24 without #undef USE_UCONTEXT.  Userland
tested on a Debian unstable-release machine.

Change-Id: I69a4226622748b18f5835d6517297b2d3750586a
Reviewed-on: http://gerrit.openafs.org/11567
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>