[dev.link] cmd/link: add loader.Loader apis for symbol dynamic id

[dev.link] cmd/link: add loader.Loader apis for symbol dynamic id

Add SymDynid and SetSymDynid methods to the loader. This symbol
property is currently backed by a map.

Change-Id: Iaf86b1d8aaa775fa102fadea30394eb8a670e0e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/224378
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>