This change implements the requirement of
old Go to build new Go on Plan 9. Also fix
the build of the new cmd/dist written in Go.

This is similar to the make.bash change in
CL 2470, but applied to make.rc for Plan 9.

Change-Id: Ifd9a3bd8658e2cee6f92b4c7f29ce86ee2a93c53
Reviewed-on: https://go-review.googlesource.com/2662Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>