syscall: follow convention for generated code comments in linux/riscv64 files

Use generated code comments following the convention
https://golang.org/s/generatedcode.

Follow-up for CL 204659

Change-Id: I7a3467ff9477488616b17f059245368d3d82d7a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/208417
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/zerrors_linux_riscv64.go

 // mkerrors.sh
-// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
+// Code generated by the command above; DO NOT EDIT.
 
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs -- _const.go

src/syscall/zsysnum_linux_riscv64.go

-// mksysnum_linux.pl /usr/include/asm/unistd.h
-// MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
+// mksysnum_linux.pl /usr/include/asm/unistd.h /usr/include/asm-generic/unistd.h
+// Code generated by the command above; DO NOT EDIT.
 
 package syscall