Update comments for duffzero and duffcopy
which referred to legacy locations:
+ cmd/?g/cgen.go
+ cmd/?g/ggen.go

Remnants of the old days when we had 5g, 6g etc.

Those locations have since moved to:
+ cmd/compile/internal/<arch>/cgen.go
+ cmd/compile/internal/<arch>/ggen.go

Change-Id: Ie2ea668559d52d42b747260ea69a6d5b3d70e859
Reviewed-on: https://go-review.googlesource.com/29073Reviewed-by: Russ Cox <rsc@golang.org>