cmd/internal/obj/riscv: improve instruction validation

Ensure that rs2 is none for various instruction encodings. Fix a couple of cases
where it should have been but is not.

Change-Id: I9f8211a0257e49643dbbc89e158e048050ebe6f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/349649


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>