[dev.link] cmd/link: remove holes from global index space

In CL 217064, we made symbol's global index unique, but we still
reserve index space for each object file, which means we may
leave holes in the index space if the symbol is a dup or is
overwritten. In this CL, we stop reserving index spaces. Instead,
symbols are added one at a time, and only added if it does not
already exist. There is no more holes in the index space.

Change-Id: I3c4e67163c556ba1198e13065706510dac4692fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/217519


Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>