[dev.cc] cmd/5c, cmd/6c, cmd/8c, cmd/cc: remove

Let's just do this up front.
This will break the build (here on the dev.cc branch).
The CLs that follow will take care of fixing it.

Leave behind cmd/cc/lexbody and cmd/cc/macbody for the assemblers.
They'll go away later.

LGTM=dave, r
R=r, dave
CC=golang-codereviews
https://codereview.appspot.com/172170043