cmd/gc contains symbol references into the back end dirs like 6g.
It also contains a few files that include the back end header files and
are compiled separately for each back end, despite being in cmd/gc.
cmd/gc also defines main, which makes at least one reverse symbol
reference unavoidable. (Otherwise you can't get into back-end code.)

This was all expedient, but it's too tightly coupled, especially for a
program written Go.

Make cmd/gc into a true library, letting the back end define main and
call into cmd/gc after making the necessary references available.
cmd/gc being a real library will ease the transition to Go.

Change-Id: I4fb9a0e2b11a32f1d024b3c56fc3bd9ee458842c
Reviewed-on: https://go-review.googlesource.com/3277Reviewed-by: Rob Pike <r@golang.org>