cmd/internal/obj/riscv: implement AUIPC and LUI instructions

Add support for assembling AUIPC and LUI instructions.

Based on the riscv-go port.

Updates #27532

Change-Id: I178868b6dcc6fdc6b8527454569a3538ed50723e
Reviewed-on: https://go-review.googlesource.com/c/go/+/196840


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>