tcg/riscv: Support ADD.UW, SEXT.B, SEXT.H, ZEXT.H from Zba+Zbb

Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>