[dev.link] cmd/link: minor tweaks to PropagateLoaderChangesToSymbols

[dev.link] cmd/link: minor tweaks to PropagateLoaderChangesToSymbols

Update PropagateLoaderChangesToSymbols so that it no longer requires
a sym.Symbols pointer. The intent is to generalize it a little to
allow it to be used in more than just linker Dwarf generation.

Change-Id: I9bddc5d39839eacd9113c945bb59d2873c0b088c
Reviewed-on: https://go-review.googlesource.com/c/go/+/224381
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>