cmd/link: allow duplicated weak symbols on Mach-O

This fixes cgo test issue29563 on Darwin.

Updates #29563.

Change-Id: If480078461247cd7c95931ae3ad4ca89736dd550
Reviewed-on: https://go-review.googlesource.com/c/go/+/170015


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>