cmd/interna/obj/riscv: use obj.REG_NONE rather than 0

No functional change, but makes the code consistent and more readable.

Change-Id: I33db681fe0ed501842755f27b6e7843cdc8877cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/345049


Trust: Joel Sing <joel@sing.id.au>
Trust: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>