cmd/genbootstrap: new tool to generate minimal GOROOT_BOOTSTRAP tarballs
cmd/genbootstrap: new tool to generate minimal GOROOT_BOOTSTRAP tarballs

Fixes golang/go#9797

Change-Id: Ib8a8a5f9c0e51c22b93a1ba2957362e8fe829937
Reviewed-on: https://go-review.googlesource.com/21657
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/build,golang/build,golang/build,golang/build