This CL gives Linkmode a type, switches it to the standard flag
handling mechanism, and deduplicates some logic.

There is a semantic change in this CL. Previously if a link was
invoked explicitly with -linkmode=internal, any condition that forced
external linking would silently override this and use external
linking. Instead it now fails with a reason why. I believe this is an
improvement, but will change it back if there's disagreement.

Fixes #12848

Change-Id: Ic80e341fff65ecfdd2b6fdd6079674cc7210fc5f
Reviewed-on: https://go-review.googlesource.com/28971
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>