cmd/6g, etc: cleanup, moving code out of near-empty gg.go files

cmd/6g, etc: cleanup, moving code out of near-empty gg.go files

Clear out gg.go files, and move things into consistent places between
the cmd/?g directories.

Change-Id: I81e04180613b806e0bfbb88519e66111ce9f74a3
Reviewed-on: https://go-review.googlesource.com/9080
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>