x509roots: generate a stable sort, for real this time

Sort based on the stringified subject, then break ties based on the raw
DER (which will, actually, be unique this time).

Change-Id: I3dd912fb19b103e92fabfb4562e31c6dcec40614
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/505695
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>

Author: rolandshoemaker