cmd/internal/obj/riscv: handle MOV rewrites in separate pass

Handle the rewriting of MOV pseudo-instructions in a separate pass. This allows
AMOV to be more readily used by other code, including AGETCALLERPC and ARET
implementations.

Updates #27532

Change-Id: Iea794121210048ed23ed0fb8f3c5e3cd5354a311
Reviewed-on: https://go-review.googlesource.com/c/go/+/212758


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