The primary build tools cmd/asm, cmd/compile, and cmd/link are
built during cmd/dist bootstrap and then assumed by cmd/go to
be available for any future builds.

The only tool invoked by cmd/go during a build and not in this list
is cmd/cgo; instead of being built during cmd/dist and assumed by
cmd/go, cmd/go arranges to build cmd/cgo if needed as part of
the regular build. We got here because at the time cmd/go was written,
cmd/cgo was the only build tool written in Go (the others were in C),
and so it made some sense to put cmd/dist in charge of building
the C tools and to have custom code in cmd/go to build cmd/cgo
just in time for it to be used by a particular build.

This custom code has historically been quite subtle, though, because
the build of cmd/cgo inherits whatever build flags apply to the
build that wants to use cmd/cgo. If you're not careful,
"go install -race strings" might under the wrong circumstances
also install a race-enabled cmd/cgo binary, which is unexpected
at the least.

The custom code is only going to get more problematic as we
move toward more precise analysis of whether dependencies are
up-to-date. In that case, "go build -race strings" will check to
see if there is not just a cmd/cgo already but a race-enabled
cmd/cgo, which makes no sense.

Instead of perpetuating the special case, treat cgo like all the
other build tools: build it first in cmd/dist, and then assume it is present.
This simplifies cmd/go.

Building cmd/cgo during bootstrap also allows the default
build of cmd/cgo to be built using cgo, which may be necessary
on future essentially-cgo-only systems.

Change-Id: I414e22c10c9920f4e98f97fa35ff22058c0f143d
Reviewed-on: https://go-review.googlesource.com/68338
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>