cmd/link: avoid crash on undefined func sym with external linking
Fix a buglet in relocation processing that crops up with external
linking when you have an undefined function symbol that also has a
prototype (as if it were being defined in assembly src).
Fixes #47993.
Change-Id: Ib655492a63b205ffdc124cfd0cb7f7b731571821
Reviewed-on: https://go-review.googlesource.com/c/go/+/345473
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com> Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Jeremy Faller <jeremy@golang.org>