cmd/internal/ld: use usual flooding for c-archive

I said I removed this from cl/8711 in response to your comment, but
apparently I did not.

misc/cgo/testcarchive continues to pass on darwin/amd64.

Change-Id: I6410782f2a78bf117741628fb71cac56e289b590
Reviewed-on: https://go-review.googlesource.com/9010Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/ld/go.go

@@ -622,7 +622,7 @@ func deadcode() {
 		fmt.Fprintf(&Bso, "%5.2f deadcode\n", obj.Cputime())
 	}
 
-	if Buildmode == BuildmodeShared || Buildmode == BuildmodeCArchive {
+	if Buildmode == BuildmodeShared {
 		// Mark all symbols as reachable when building a
 		// shared library.
 		for s := Ctxt.Allsym; s != nil; s = s.Allsym {