[dev.link] cmd/link: ignore undefined symbols in trampoline generation

Don't emit an error for undefined symbol in trampoline
generation pass, which will be duplicate as we'll emit a better
one later.

Fix TestUndefinedRelocErrors on PPC64.

Change-Id: I964d4bd63ec4b4c6eb5d98caf68db93ce6488bf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/227617


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>