[dev.link] cmd/link: allow either of duplicated symbols being dupok

[dev.link] cmd/link: allow either of duplicated symbols being dupok

If two symbols have the same name, the old code allows either one
being dupok (preferably both, but either is ok). Currently, the
new code only works when the new symbol being dupok (or both).
Allow only old symbol being dupok as well.

One example for this is the tls_g variable on ARM64 and PPC64
when the race detector is enabled.

Should fix Linux/ARM64 build.

Change-Id: I8dd21c017e826847f13471c30dfd71bf225d8076
Reviewed-on: https://go-review.googlesource.com/c/go/+/201642
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>