[dev.link] cmd/link: check fingerprint for index consistency

[dev.link] cmd/link: check fingerprint for index consistency

Previous CL introduced index fingerprint in the object files.
This CL implements the second part: checking fingerprint
consistency in the linker when packages are loaded.

Change-Id: I05dd4c4045a65adfd95e77b625d6c75a7a70e4f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/229618
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>