runtime: update mkduff legacy comments

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/29073
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mkduff.go

@@ -8,14 +8,14 @@
 // The compiler jumps to computed addresses within
 // the routine to zero chunks of memory.
 // Do not change duffzero without also
-// changing clearfat in cmd/?g/ggen.go.
+// changing the uses in cmd/compile/internal/*/*.go.
 
 // runtime·duffcopy is a Duff's device for copying memory.
 // The compiler jumps to computed addresses within
 // the routine to copy chunks of memory.
 // Source and destination must not overlap.
 // Do not change duffcopy without also
-// changing blockcopy in cmd/?g/cgen.go.
+// changing the uses in cmd/compile/internal/*/*.go.
 
 // See the zero* and copy* generators below
 // for architecture-specific comments.