[dev.ssa] cmd/compile: improve stability of generated code

[dev.ssa] cmd/compile: improve stability of generated code

If the files in cmd/compile/internal/ssa/gen
are passed to go run in a different order,
e.g. due to shell differences or manual entry,
then the order of constants in opGen churns.

Sort archs by name to enforce stability.
The movement of the PPC constants is a one time cost.

Change-Id: Iebcfdb9e612d7dd8cde575f920f1292891f2f24a
Reviewed-on: https://go-review.googlesource.com/24680
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>