cmd/dist: ensure android-implies-linux for file names in dist

cmd/dist: ensure android-implies-linux for file names in dist

This is one hurdle to building Go on Android; the runtime does
not build properly because *_linux.go files are excluded from
the "Building go_bootstrap" step when GOOS=android.

There are other hurdles; this is the first one.

Change-Id: I766e4bbf6ffc0d273888913f2516cf3e995a1786
Reviewed-on: https://go-review.googlesource.com/38308
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>