cmd/cgo is special among the build tools because it is (re)built on demand
when needed for a package using cgo, to avoid additional bootstrap logic
elsewhere. (This is in contrast to cmd/compile, cmd/link, and so on, which
must be specially built before even invoking the go command.)

When the go command starts using content-based decisions about staleness,
it is important that the build of cmd/cgo never use -linkmode=external,
because that depends on runtime/cgo, which in turn depends on cmd/cgo.

Change-Id: I72a2be748606d1ed4b93a54f2a5c7084e87d5fbc
Reviewed-on: https://go-review.googlesource.com/67310Reviewed-by: Ian Lance Taylor <iant@golang.org>