[dev.link] cmd/link: implement symbol overwrite logic

[dev.link] cmd/link: implement symbol overwrite logic

If two defined symbols have the same name, one contentless and
one with content, the one with content "wins". This is mainly for
go:linkname on data symbols. Support this logic in newobj mode.

Introduce an "overwrite" mechanism, letting one symbol overwrite
another. This machanism could later be used for the linker
overwriting symbol contents (e.g. -X flag).

Change-Id: I32ee7d4b82df275f11b38c3abefc99b878ff12d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/200097
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>