cmd/internal/obj: add SortSlice

cmd/internal/obj: add SortSlice

sort.Slice was added in Go 1.8.
It's nice to use, and faster than sort.Sort,
so it'd be nice to be able to use it in the toolchain.
This CL adds obj.SortSlice, which is sort.Slice,
but with a slower fallback version for bootstrapping.

This CL also includes a single demo+test use.

Change-Id: I2accc60b61f8e48c8ab4f1a63473e3b87af9b691
Reviewed-on: https://go-review.googlesource.com/40114
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>