A followup CL will rewrite listsort to use the new cmpstackvarlt and
change cmpstackvar to avoid stringsCompare.

Change-Id: Idf0857a3bd67f9e2243ba82aa0bff510612927c3
Reviewed-on: https://go-review.googlesource.com/14611Reviewed-by: Dave Cheney <dave@cheney.net>