cmd/internal/obj/riscv: avoid obj.Prog rewriting for memory loads

Rather than rewriting the obj.Prog for a MOV pseudo-instruction targeting
a memory to register load, generate the appropriate machine instruction
sequence directly.

Change-Id: I4c7292ba00f576ec71d4842b6ff27a8ce6db0650
Reviewed-on: https://go-review.googlesource.com/c/go/+/344454


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>