cmd/go: permit .tbd files as a linker flag

A .tbd file is a macOS text-based stub library and is a valid input to
the macOS linker. This change adds .tbd to the allow-list for acceptable
linker flags.

Fixes golang/go#44263

Change-Id: Ie5439a13325dbc908e42f95ec70aca518bb549f9
GitHub-Last-Rev: 6055c3b5fa8c8146c0c5c976aa8e1d219e01e414
GitHub-Pull-Request: golang/go#44276
Reviewed-on: https://go-review.googlesource.com/c/go/+/292269


Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>