cmd/go: use the last -linkmode flag to determine external linking

Current linkmode checking in determining package dependencies doesn't
take multiple -linkmode options into consideration, may lead to redundant
dependency on 'runtime/cgo'.

Fixes the problem and adds a testcase.

Change-Id: Iac5ea9fb3ca5ef931201afd0f3441f41f946c919
Reviewed-on: https://go-review.googlesource.com/c/go/+/263497


Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>