Bootstrap the Go parts of the Go toolchain using Go 1.4,
as described in https://golang.org/s/go15bootstrap.

The first Go part of the Go toolchain will be cmd/objwriter,
but for now that's just an empty program to test that this
new code works.

Once the build dashboard is okay with this change,
we'll make objwriter a real program depended upon by the build.

Change-Id: Iad3dce675571cbdb5ab6298fe6f98f53ede47d5c
Reviewed-on: https://go-review.googlesource.com/3044Reviewed-by: Ian Lance Taylor <iant@golang.org>