plan9: follow convention for generated code comment

Follow the convertion (https://golang.org/s/generatedcode) for generated
code.

Change-Id: If06c4d5c3974a163ff75a9acc7141292eb43b7ef
Reviewed-on: https://go-review.googlesource.com/118818
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

plan9/mkerrors.sh

@@ -140,7 +140,7 @@ echo '#include <signal.h>' | $CC -x c - -E -dM $ccflags |
 	sort >_signal.grep
 
 echo '// mkerrors.sh' "$@"
-echo '// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT'
+echo '// Code generated by the command above; DO NOT EDIT.'
 echo
 go tool cgo -godefs -- "$@" _const.go >_error.out
 cat _error.out | grep -vf _error.grep | grep -vf _signal.grep

plan9/mksyscall.pl

@@ -308,7 +308,7 @@ if($errors) {
 
 print <<EOF;
 // $cmdline
-// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
+// Code generated by the command above; DO NOT EDIT.
 
 package plan9
 

plan9/zsyscall_plan9_386.go

 // mksyscall.pl -l32 -plan9 syscall_plan9.go
-// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
+// Code generated by the command above; DO NOT EDIT.
 
 package plan9
 

plan9/zsyscall_plan9_amd64.go

 // mksyscall.pl -l32 -plan9 syscall_plan9.go
-// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
+// Code generated by the command above; DO NOT EDIT.
 
 package plan9
 

plan9/zsyscall_plan9_arm.go

 // mksyscall.pl -l32 -plan9 -tags plan9,arm syscall_plan9.go
-// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
+// Code generated by the command above; DO NOT EDIT.
 
 // +build plan9,arm