target/riscv: Add a riscv_cpu_is_32bit() helper function

target/riscv: Add a riscv_cpu_is_32bit() helper function

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Message-id: ebd37b237a8cbe457335b948bd57f487b6b31869.1608142916.git.alistair.francis@wdc.com