cmd/dist: make textflag.h available in runtime, avoid android/linux conflicts

1) cmd/dist was copying textflag.h to the build include directory,
but only after compiling package runtime. So other packages could
use it, just not runtime. Copy earlier, so that runtime can use it too.

2) We decided for android that anything marked linux is also included
in the build. The generated linux-specific files in cmd/dist must therefore
have explicit +build !android tags, or else you can't have simultaneous
linux/arm and android/arm builds in a single client. The tag was already
there for at least one file, but it was missing from many others.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/134500043