cmd/compile: add debugging mode for import/export

Just add a simple magic number with each op, to detect when
the reader gets desynchronized from the writer.

Change-Id: Iac7dab7f465b0021b1d7ae31c8f8a353ac3663a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/299769


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>