[dev.regabi] cmd/compile: iexport debug crumbs for toolstash

Prints offsets for declarations, inline bodies, and strings when -v is
used. Still not much, but hopefully useful for narrowing down the
cause of export data differences.

Change-Id: I9b2e4a3d55b92823fa45a39923e8c4b25303693c
Reviewed-on: https://go-review.googlesource.com/c/go/+/276112


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>