cmd/internal/obj/riscv: avoid obj.Prog rewriting for store instructions

Rather than rewriting the obj.Prog for a store instruction, generate the
appropriate machine instruction sequence directly.

Change-Id: I026250ca3d249a1013243948a4e567b708b72d0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/344452


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>