[dev.regabi] cmd/compile: separate exportsym more cleanly

Clean up a TODO (and make the package gc split easier)
by moving the exportsym walk out of iexport proper.

Also move exportsym call out of fninit.

Change-Id: Ie5887a68d325f7154201f4a35b9b4be4bf4b48dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/279298


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