[dev.link] cmd/link: support marking outer/sub for external loader.Sym
Add a loader mechanism for recording outer/sub relationships between
symbols without falling back on sym.Symbol. Also includes a new
"PrependSub" method that provides a way to chain a sub-symbol only the
list of an outer symbol (a common operation when manipulating
outer/sub relationships in the linker).
Change-Id: I70c72356945ceec2bacdcdc25bcc352bfb6765a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/210777
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Cherry Zhang <cherryyz@google.com> Reviewed-by: Jeremy Faller <jeremy@golang.org>