Make cmd/ld a real library invoked by the individual linkers.
There are no reverse symbol references anymore
(symbols referred to in cmd/ld but defined in cmd/5l etc).

This means that in principle we could do an automatic
conversion of these to Go, as a stopgap until cmd/link is done
or as a replacement for cmd/link.

Change-Id: I4a94570257a3a7acc31601bfe0fad9dea0aea054
Reviewed-on: https://go-review.googlesource.com/4649Reviewed-by: Rob Pike <r@golang.org>