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: I3cb2477b2ad4baa7c2007323b983eb29404b0aac
Reviewed-on: https://go-review.googlesource.com/c/go/+/192597


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>