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/gsubr.go b/src/cmd/8g/gsubr.go
index a5ee8ed..34ddfe0 100644
--- a/src/cmd/8g/gsubr.go
+++ b/src/cmd/8g/gsubr.go
@@ -42,6 +42,13 @@
 // At the same time, can raise StackBig in ../../runtime/stack.h.
 var unmappedzero uint32 = 4096
 
+// foptoas flags
+const (
+	Frev  = 1 << 0
+	Fpop  = 1 << 1
+	Fpop2 = 1 << 2
+)
+
 /*
  * return Axxx for Oxxx on type t.
  */