build: require old Go to build new Go (and convert cmd/dist to Go)

This CL introduces the bootstrap requirement that in order to
build the current release (or development version) of Go, you
need an older Go release (1.4 or newer) already installed.
This requirement is the whole point of this CL.

To enforce the requirement, convert cmd/dist from C to Go.

With this bootstrapping out of the way, we can move on to
replacing other, larger C programs like the Go compiler,
the assemblers, and the linker.

See golang.org/s/go15bootstrap for details.

Change-Id: I53fd08ddacf3df9fae94fe2c986dba427ee4a21d
Reviewed-on: https://go-review.googlesource.com/2470


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>