[dev.link] cmd/link: fix aux symbol handling in Funcdata

If a Go symbol is cloned to external, we should preserve its Aux
symbols for FuncInfo, etc.. We already do this in
loader.FuncInfo, but not in FuncInfo.Funcdata. Do it in the
latter as well. In fact, since FuncInfo and Funcdata should use
the same set of auxs, just record the auxs and reuse.

Should fix PPC64 build.

Change-Id: Iab9020eaca15d98fe3bb41f50f0d5bdb4999e8c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/227848


Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>