A contentless data symbol may be a declaration of a cgo-imported
variable, e.g.

	//go:cgo_import_dynamic xxx
	var xxx uintptr

In this case, we want to mark the symbol imported, instead of
defined with zero value.

We used to load cgo directives before loading the object file, so
we'll mark the symbol SDYNIMPORT first. But in newobj mode,
currently we load cgo directives later. Letting SDYNIMPORT
overwrite contentless data symbol makes it work in both ordering.

Change-Id: I878f52086d6cdb5a347669bf8f848a49bce87b52
Reviewed-on: https://go-review.googlesource.com/c/go/+/201020
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>