cmd/link: skip TestOldLink if the old linker does not exist

cmd/link: skip TestOldLink if the old linker does not exist

We don't ship the old linker in binary releases. Skip the test if
we cannot find the old linker.

Fixes #39509.

Change-Id: I1af5552bc56aff5314a384bcb5f3717b725d68e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/242604
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>