cmd/internal/obj/riscv: simplify validation and encoding of raw instructions

cmd/internal/obj/riscv: simplify validation and encoding of raw instructions

Use wantImmU/immU rather than handrolling the same code. This also corrects
the validation output - add tests to ensure this is the case.

Change-Id: Id48f459c7c0de09ddde7a10506f66a3a269f325f
Reviewed-on: https://go-review.googlesource.com/c/go/+/702396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>