files for unsupported architectures are deleted, as it would require
changing cmd/dist to recognize their names as build tags (probably
need a separated CL).

Change-Id: Ifd164b014867d39b4924d1b859fb84317dce4ab0
Reviewed-on: https://go-review.googlesource.com/14928Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>