[dev.link] cmd/link: fold zero symbol check into ResolveABIAlias

[dev.link] cmd/link: fold zero symbol check into ResolveABIAlias

We call (or will call) ResolveABIAlias in many places. Doing zero
symbol check everytime is annoying. Fold the condition into
ResolveABIAlias.

Change-Id: I10485fe83b9cce2d19b6bd17dc42176f72dae48b
Reviewed-on: https://go-review.googlesource.com/c/go/+/231046
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>