cmd/link: skip symbol references when looking for missing symbols

ErrorUnresolved attempts to find the missing symbol in another ABI,
in order to provide more friendly error messages. However, in doing so
it checks the same ABI and can find the symbol reference for the symbol
that it is currently reporting the unresolved error for. Avoid this by
ignoring SXREF symbols, which is the same behaviour used when linking
is performed.

Fixes #33979

Change-Id: I9bfc40146dec2666d25e93d3bcd1984da5c71215
Reviewed-on: https://go-review.googlesource.com/c/go/+/220917


Run-TryBot: Than McIntosh <thanm@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>