cmd/compile: separate out parameter and field export encoding

These two types of *types.Field encode different concepts, so we
encode them separately (and ignore fields that don't matter for
each concept).

Change-Id: I9d1608413949a109f12a3ebd52cd7af5f476e415
Reviewed-on: https://go-review.googlesource.com/c/go/+/312130


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>