cmd/interna/obj/riscv: use obj.REG_NONE rather than 0
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 <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Trust: Meng Zhuo <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Meng Zhuo <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>

golang/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,golang/go,cloudflare/go