[dev.link] cmd/link: restore export data hash

[dev.link] cmd/link: restore export data hash

With the previous CL, the export data will not change whether it
is compiled with -dynlink flag or not. Restore the export data
hash, and reenable plugin version check.

TODO: it may be still better to just generate a fingerprint for
each package at compile time.

Change-Id: I1f298ac97c3ab9b8d05d1c95e8be74d10ca7cd0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/201720
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>