Simplify determineLinkMode by calling mustLinkExternal upfront,
then doing a first pass for LinkModeAuto, followed by a second pass
that determines if the link mode is valid.
Change-Id: I9d7668107c159f8fe330b8c05fee035bbe9875fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/195078 Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>