[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>

misc/cgo/testplugin/testdata/host/host.go

@@ -145,13 +145,12 @@ func main() {
 	}
 
 	_, err = plugin.Open("plugin-mismatch.so")
-	// TODO: newobj
-	//if err == nil {
-	//	log.Fatal(`plugin.Open("plugin-mismatch.so"): should have failed`)
-	//}
-	//if s := err.Error(); !strings.Contains(s, "different version") {
-	//	log.Fatalf(`plugin.Open("plugin-mismatch.so"): error does not mention "different version": %v`, s)
-	//}
+	if err == nil {
+		log.Fatal(`plugin.Open("plugin-mismatch.so"): should have failed`)
+	}
+	if s := err.Error(); !strings.Contains(s, "different version") {
+		log.Fatalf(`plugin.Open("plugin-mismatch.so"): error does not mention "different version": %v`, s)
+	}
 
 	_, err = plugin.Open("plugin2-dup.so")
 	if err == nil {

src/cmd/link/internal/ld/lib.go

@@ -816,7 +816,7 @@ func genhash(ctxt *Link, lib *sym.Library) {
 		return
 	}
 	h.Write(pkgDefBytes[0:firstEOL])
-	//h.Write(pkgDefBytes[firstDoubleDollar : firstDoubleDollar+secondDoubleDollar]) // TODO: newobj: -dynlink may change symbol numbering? which will make the export data differ
+	h.Write(pkgDefBytes[firstDoubleDollar : firstDoubleDollar+secondDoubleDollar])
 	lib.Hash = hex.EncodeToString(h.Sum(nil))
 }