[dev.link] cmd/link: expand set of symbol attributes in loader
Add in a collection of new loader interfaces for getting/setting
symbol attributes, e.g. properties that would normally be part of the
sym.Symbol "Attr" field. This change also moves references to the
loaders 'reachable' bitmap behind a pair of loader methods, so that we
a consistent way of accessing symbol attributes overall. It is worth
noting that not every symbol attribute is backed by a bitmap; for some
infrequently used attributes, a map[Sym]struct{} is used instead.
Change-Id: I0010c9cd928d41b4bb6cdf45db4581e11c3c5db3
Reviewed-on: https://go-review.googlesource.com/c/go/+/210778
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>