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://golang.org/cl/172170043