Remove support for Solaris pre-8

Remove support for Solaris pre-8

Remove support for all Solaris and SunOS platforms prior to Solaris 8,
since Solaris 7 reached end-of-life in August of 2008. Remove all
non-documentation references to sunx86_57 and earlier, sun4x_57 and
earlier, and AFS_SUN57_ENV and earlier.

References to AFS_SUN58_ENV have been changed to AFS_SUN5_ENV where
appropriate, and AFS_SUN5_ENV now implies Solaris 8.
AFS_SUN57_64BIT_ENV has been renamed to AFS_SUN5_64BIT_ENV.

Change-Id: Ia64ce7da7bfc685fa28a5119c51ec740625456e3
Reviewed-on: http://gerrit.openafs.org/4888
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>