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>
diff --git a/src/cmd/8g/ggen.go b/src/cmd/8g/ggen.go
index 21191ed..5902552 100644
--- a/src/cmd/8g/ggen.go
+++ b/src/cmd/8g/ggen.go
@@ -189,6 +189,8 @@
 	}
 }
 
+var panicdiv *gc.Node
+
 /*
  * generate division.
  * caller must set: