Currently zosarch.go is written out in non-deterministic map order.
Sort the keys and write it out in sorted order to make the generated
file contents deterministic.

Change-Id: Id490f0e8665a2c619c5a7a00a30f4fc64f333258
Reviewed-on: https://go-review.googlesource.com/24174
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>