tcg/riscv: Fix StoreStore barrier generation

tcg/riscv: Fix StoreStore barrier generation

On RISC-V to StoreStore barrier corresponds
`fence w, w` not `fence r, r`

Cc: qemu-stable@nongnu.org
Fixes: efbea94c76b ("tcg/riscv: Add slowpath load and store instructions")
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Denis Tomashev <denis.tomashev@syntacore.com>
Signed-off-by: Roman Artemev <roman.artemev@syntacore.com>
Message-ID: <e2f2131e294a49e79959d4fa9ec02cf4@syntacore.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>