cmd/compile: update a few stale comments

CL 64811 removed dcopy. Update the comment in types.Sym.

The Russquake moved iexport.go. Update the path to it.

WRAPPER is now also used by ABI wrappers, so update the comment since
it's now more general than method wrappers.

Change-Id: Ie0df61dcef7168f6720838cd5c9a66adf546a44f
Reviewed-on: https://go-review.googlesource.com/c/go/+/305269


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>