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

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

Change-Id: I6eac8637e1fcb48c04d9f331cac7ae5d8cc54092
Reviewed-on: https://go-review.googlesource.com/c/go/+/344455


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