[dev.link] cmd/link: remove Sub field from sym.Symbol

[dev.link] cmd/link: remove Sub field from sym.Symbol

Remove 'Sub' field from sym.Symbol, replacing uses (those downstream of
loadlibfull) with loader method calls.

NB: removing the Outer field will have to wait for now; it is accessed
in archreloc methods that don't have access to link ctxt or loader
currently.

Change-Id: I2abe5906fc169c64b2ab7d5ad213619bea5a17c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/230617
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>