cmd/link: adjust whitelist for -strictdups checking for plan9

Add a couple of additional entries to the white list used to screen
out errors for builtin functions; these correspond to cases
that appear to come up only on the plan9 builder.

Updates #31503.

Change-Id: I48ab942ab2894240efe651ec7b7eace7aa5cb45e
Reviewed-on: https://go-review.googlesource.com/c/go/+/172986


Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>