cmd/internal/obj/riscv: implement MOV pseudo-instructions

Add support for rewriting MOV pseudo-instructions into appropriate
RISC-V instructions.

Based on the riscv-go port.

Updates #27532

Change-Id: I22da6f5f12c841d56fb676ab2a37f6d0a686b033
Reviewed-on: https://go-review.googlesource.com/c/go/+/198677


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