[dev.link] cmd/link: propagate gotype symbol info in cloneToExternal

[dev.link] cmd/link: propagate gotype symbol info in cloneToExternal

This patch fixes a problem with the loader's cloneToExternal method,
specifically that the new external clone created did not get
the proper Gotype value from its orginal symbol.

Change-Id: I9978140d285104d407bf55649fb6ed94959933f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/213639
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>