target/riscv: Add pointer masking tb flags

Signed-off-by: Alexey Baturo <baturo.alexey@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20250106102346.1100149-5-baturo.alexey@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>