On linux/riscv64, the renameat syscall no longer exists and has been
superseded by renameat2. Thus we'll have to use Renameat2 to implement
Renameat on linux/riscv64 for #27532. Prepare for this by moving the
Renameat definition to the GOARCH specific files.

Follow CL 157899 which did the same for golang.org/x/sys/unix

Change-Id: I9670213cc3987df48fee962ddee36915a7785560
Reviewed-on: https://go-review.googlesource.com/c/go/+/192077
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>