[dev.link] cmd/link: simplify field tracking support

[dev.link] cmd/link: simplify field tracking support

Currently, for the special field tracking symbol go.track.XXX,
when they are reachable, we set its type to SCONST. There is no
need to do that. Just leave it unset (as Sxxx). The symbol is
done after this point.

Change-Id: I966d80775008f7fb5d30fbc6b9e4a30ae8316b6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/233998
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>