fix Base.Sort.Float.issignleft bug: 0.0 before -0.0 in reverse mode.
sortperm: one way to fix #6177, but with 34% slowdown.

The approach here is to change the Perm comparison to break ties
by index values. This allows using an unstable algorithm getting
stable sortperm results but kills performance despite the switch
to quicksort.
Merge branch 'master' of github.com:JuliaLang/julia