[dev.link] cmd/link: add dynimp, localentry attributes for loader.Sym
Add new methods to get/set the "dynimplib", "dynimpvers" and
"localentry" attributes for an external Sym in loader.Loader. These
attribute values are stored sparsely, since we expect that most
symbols will not need them; they are set when processing cgo
directives and when dealing with host object symbols.
Change-Id: If0b3c173307801d39cb576bb99c83b9081c42d9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/211298 Reviewed-by: Jeremy Faller <jeremy@golang.org>