cmd/internal/obj/riscv: avoid obj.Prog rewriting for store instructions
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 <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go